Tracing: delete dangling symlinks before creating new ones.
Tracing: delete dangling symlinks before creating new ones.

Summary:
6ab95780 fixed the tracing.py script to work for windows, but also
switched to recreating symlink by first using ##os.path.exists()## to
check if an old version of the symlink needs to be cleaned up before
creating the new one.  However, ##os.path.exists()## uses the ##stat##
syscall under the covers which follows symlinks, and returns ##False##
if the symlink is dangling.

This diff switches to use ##os.path.lexists()##, which uses ##lstat##,
to avoid following the symbolic link, so that we'll still cleanup
dangling symlinks.

Test Plan: arc diff

shs96c/buck,bocon13/buck,darkforestzero/buck,romanoid/buck,zpao/buck,clonetwin26/buck,Heart2009/buck,rmaz/buck,facebook/buck,shybovycha/buck,stuhood/buck,lukw00/buck,mogers/buck,janicduplessis/buck,darkforestzero/buck,artiya4u/buck,mread/buck,SeleniumHQ/buck,zhan-xiong/buck,robbertvanginkel/buck,tgummerer/buck,robbertvanginkel/buck,raviagarwal7/buck,sdwilsh/buck,robbertvanginkel/buck,LegNeato/buck,davido/buck,illicitonion/buck,mogers/buck,janicduplessis/buck,tgummerer/buck,vschs007/buck,artiya4u/buck,kageiit/buck,SeleniumHQ/buck,dpursehouse/buck,Learn-Android-app/buck,k21/buck,vine/buck,sdwilsh/buck,daedric/buck,ilya-klyuchnikov/buck,brettwooldridge/buck,robbertvanginkel/buck,mikekap/buck,illicitonion/buck,Dominator008/buck,liuyang-li/buck,daedric/buck,mnuessler/buck,SeleniumHQ/buck,grumpyjames/buck,zhuxiaohao/buck,pwz3n0/buck,artiya4u/buck,ilya-klyuchnikov/buck,brettwooldridge/buck,LegNeato/buck,rhencke/buck,SeleniumHQ/buck,grumpyjames/buck,tgummerer/buck,davido/buck,brettwooldridge/buck,stuhood/buck,raviagarwal7/buck,daedric/buck,artiya4u/buck,sdwilsh/buck,rowillia/buck,marcinkwiatkowski/buck,Dominator008/buck,Heart2009/buck,Distrotech/buck,luiseduardohdbackup/buck,k21/buck,grumpyjames/buck,ilya-klyuchnikov/buck,vschs007/buck,Dominator008/buck,romanoid/buck,mikekap/buck,robbertvanginkel/buck,Addepar/buck,MarkRunWu/buck,Addepar/buck,bocon13/buck,illicitonion/buck,luiseduardohdbackup/buck,vschs007/buck,siddhartharay007/buck,rhencke/buck,pwz3n0/buck,stuhood/buck,illicitonion/buck,mikekap/buck,brettwooldridge/buck,darkforestzero/buck,clonetwin26/buck,justinmuller/buck,robbertvanginkel/buck,shybovycha/buck,ilya-klyuchnikov/buck,pwz3n0/buck,siddhartharay007/buck,kageiit/buck,stuhood/buck,robbertvanginkel/buck,SeleniumHQ/buck,Distrotech/buck,marcinkwiatkowski/buck,liuyang-li/buck,hgl888/buck,LegNeato/buck,facebook/buck,rowillia/buck,k21/buck,romanoid/buck,lukw00/buck,mread/buck,Addepar/buck,dushmis/buck,daedric/buck,Distrotech/buck,liuyang-li/buck,rmaz/buck,OkBuilds/buck,justinmuller/buck,LegNeato/buck,janicduplessis/buck,vine/buck,mikekap/buck,zhan-xiong/buck,nguyentruongtho/buck,tgummerer/buck,zhan-xiong/buck,raviagarwal7/buck,luiseduardohdbackup/buck,mnuessler/buck,daedric/buck,Dominator008/buck,hgl888/buck,LegNeato/buck,hgl888/buck,JoelMarcey/buck,facebook/buck,Heart2009/buck,hgl888/buck,k21/buck,Heart2009/buck,rowillia/buck,raviagarwal7/buck,janicduplessis/buck,janicduplessis/buck,clonetwin26/buck,vschs007/buck,mogers/buck,Addepar/buck,mnuessler/buck,dpursehouse/buck,stuhood/buck,mogers/buck,shybovycha/buck,janicduplessis/buck,Learn-Android-app/buck,mikekap/buck,davido/buck,SeleniumHQ/buck,liuyang-li/buck,SeleniumHQ/buck,Heart2009/buck,MarkRunWu/buck,Distrotech/buck,davido/buck,shs96c/buck,bocon13/buck,daedric/buck,marcinkwiatkowski/buck,davido/buck,rmaz/buck,illicitonion/buck,mread/buck,dpursehouse/buck,vine/buck,lukw00/buck,shybovycha/buck,justinmuller/buck,MarkRunWu/buck,clonetwin26/buck,dsyang/buck,davido/buck,rmaz/buck,neonichu/buck,rhencke/buck,rmaz/buck,illicitonion/buck,raviagarwal7/buck,siddhartharay007/buck,Addepar/buck,JoelMarcey/buck,romanoid/buck,illicitonion/buck,SeleniumHQ/buck,zpao/buck,dushmis/buck,facebook/buck,ilya-klyuchnikov/buck,1yvT0s/buck,brettwooldridge/buck,1yvT0s/buck,hgl888/buck,zhuxiaohao/buck,dpursehouse/buck,1yvT0s/buck,rowillia/buck,daedric/buck,luiseduardohdbackup/buck,bocon13/buck,shs96c/buck,Heart2009/buck,kageiit/buck,dushmis/buck,sdwilsh/buck,Distrotech/buck,clonetwin26/buck,Addepar/buck,brettwooldridge/buck,lukw00/buck,sdwilsh/buck,justinmuller/buck,k21/buck,mnuessler/buck,lukw00/buck,clonetwin26/buck,hgl888/buck,shs96c/buck,zhan-xiong/buck,lukw00/buck,daedric/buck,pwz3n0/buck,marcinkwiatkowski/buck,zhan-xiong/buck,rowillia/buck,grumpyjames/buck,nguyentruongtho/buck,shybovycha/buck,1yvT0s/buck,marcinkwiatkowski/buck,JoelMarcey/buck,mnuessler/buck,vine/buck,daedric/buck,robbertvanginkel/buck,Heart2009/buck,justinmuller/buck,shs96c/buck,Heart2009/buck,LegNeato/buck,ilya-klyuchnikov/buck,neonichu/buck,davido/buck,OkBuilds/buck,raviagarwal7/buck,JoelMarcey/buck,zhan-xiong/buck,neonichu/buck,daedric/buck,LegNeato/buck,sdwilsh/buck,davido/buck,sdwilsh/buck,bocon13/buck,davido/buck,illicitonion/buck,brettwooldridge/buck,artiya4u/buck,dushmis/buck,illicitonion/buck,mnuessler/buck,rhencke/buck,rmaz/buck,bocon13/buck,janicduplessis/buck,1yvT0s/buck,clonetwin26/buck,grumpyjames/buck,mogers/buck,janicduplessis/buck,k21/buck,k21/buck,rmaz/buck,rhencke/buck,neonichu/buck,Dominator008/buck,bocon13/buck,marcinkwiatkowski/buck,nguyentruongtho/buck,MarkRunWu/buck,romanoid/buck,zhuxiaohao/buck,OkBuilds/buck,rmaz/buck,lukw00/buck,mnuessler/buck,mnuessler/buck,OkBuilds/buck,mread/buck,OkBuilds/buck,Dominator008/buck,mikekap/buck,MarkRunWu/buck,darkforestzero/buck,OkBuilds/buck,grumpyjames/buck,zpao/buck,zhuxiaohao/buck,zhan-xiong/buck,romanoid/buck,nguyentruongtho/buck,MarkRunWu/buck,Learn-Android-app/buck,shs96c/buck,siddhartharay007/buck,marcinkwiatkowski/buck,mogers/buck,liuyang-li/buck,romanoid/buck,illicitonion/buck,rhencke/buck,dushmis/buck,MarkRunWu/buck,zhan-xiong/buck,JoelMarcey/buck,k21/buck,lukw00/buck,romanoid/buck,stuhood/buck,JoelMarcey/buck,tgummerer/buck,darkforestzero/buck,liuyang-li/buck,rmaz/buck,romanoid/buck,rowillia/buck,mikekap/buck,artiya4u/buck,janicduplessis/buck,pwz3n0/buck,Addepar/buck,dsyang/buck,siddhartharay007/buck,OkBuilds/buck,davido/buck,OkBuilds/buck,siddhartharay007/buck,pwz3n0/buck,siddhartharay007/buck,pwz3n0/buck,OkBuilds/buck,raviagarwal7/buck,robbertvanginkel/buck,shs96c/buck,zpao/buck,ilya-klyuchnikov/buck,JoelMarcey/buck,siddhartharay007/buck,zhan-xiong/buck,raviagarwal7/buck,darkforestzero/buck,Dominator008/buck,vine/buck,facebook/buck,vine/buck,dsyang/buck,justinmuller/buck,mogers/buck,vine/buck,SeleniumHQ/buck,k21/buck,janicduplessis/buck,janicduplessis/buck,dushmis/buck,neonichu/buck,ilya-klyuchnikov/buck,JoelMarcey/buck,dsyang/buck,luiseduardohdbackup/buck,OkBuilds/buck,darkforestzero/buck,sdwilsh/buck,marcinkwiatkowski/buck,SeleniumHQ/buck,artiya4u/buck,brettwooldridge/buck,rowillia/buck,nguyentruongtho/buck,stuhood/buck,shybovycha/buck,Learn-Android-app/buck,vschs007/buck,shs96c/buck,clonetwin26/buck,justinmuller/buck,mikekap/buck,mogers/buck,artiya4u/buck,illicitonion/buck,daedric/buck,siddhartharay007/buck,Addepar/buck,romanoid/buck,tgummerer/buck,zhan-xiong/buck,Distrotech/buck,artiya4u/buck,vine/buck,rhencke/buck,stuhood/buck,Learn-Android-app/buck,k21/buck,mogers/buck,Dominator008/buck,raviagarwal7/buck,dushmis/buck,vschs007/buck,raviagarwal7/buck,LegNeato/buck,vschs007/buck,mikekap/buck,dsyang/buck,Learn-Android-app/buck,sdwilsh/buck,vschs007/buck,pwz3n0/buck,OkBuilds/buck,ilya-klyuchnikov/buck,Distrotech/buck,brettwooldridge/buck,clonetwin26/buck,stuhood/buck,brettwooldridge/buck,MarkRunWu/buck,vschs007/buck,rhencke/buck,Addepar/buck,vschs007/buck,Learn-Android-app/buck,justinmuller/buck,rowillia/buck,vschs007/buck,justinmuller/buck,liuyang-li/buck,artiya4u/buck,facebook/buck,rhencke/buck,hgl888/buck,Dominator008/buck,zhan-xiong/buck,1yvT0s/buck,kageiit/buck,rhencke/buck,davido/buck,zhan-xiong/buck,Heart2009/buck,zhan-xiong/buck,Distrotech/buck,zpao/buck,mikekap/buck,darkforestzero/buck,dpursehouse/buck,zhuxiaohao/buck,zhuxiaohao/buck,mnuessler/buck,justinmuller/buck,nguyentruongtho/buck,lukw00/buck,rmaz/buck,ilya-klyuchnikov/buck,kageiit/buck,rmaz/buck,dsyang/buck,janicduplessis/buck,darkforestzero/buck,shybovycha/buck,pwz3n0/buck,OkBuilds/buck,bocon13/buck,mread/buck,nguyentruongtho/buck,liuyang-li/buck,luiseduardohdbackup/buck,liuyang-li/buck,dsyang/buck,rowillia/buck,Addepar/buck,illicitonion/buck,Addepar/buck,kageiit/buck,neonichu/buck,justinmuller/buck,lukw00/buck,facebook/buck,grumpyjames/buck,Distrotech/buck,neonichu/buck,robbertvanginkel/buck,davido/buck,dushmis/buck,ilya-klyuchnikov/buck,marcinkwiatkowski/buck,rhencke/buck,romanoid/buck,JoelMarcey/buck,dsyang/buck,k21/buck,ilya-klyuchnikov/buck,dushmis/buck,Learn-Android-app/buck,SeleniumHQ/buck,vschs007/buck,darkforestzero/buck,dsyang/buck,artiya4u/buck,dsyang/buck,grumpyjames/buck,sdwilsh/buck,LegNeato/buck,mikekap/buck,tgummerer/buck,shs96c/buck,bocon13/buck,zhuxiaohao/buck,dpursehouse/buck,stuhood/buck,marcinkwiatkowski/buck,darkforestzero/buck,LegNeato/buck,hgl888/buck,raviagarwal7/buck,SeleniumHQ/buck,stuhood/buck,marcinkwiatkowski/buck,clonetwin26/buck,shybovycha/buck,Distrotech/buck,rowillia/buck,raviagarwal7/buck,Dominator008/buck,Learn-Android-app/buck,robbertvanginkel/buck,bocon13/buck,sdwilsh/buck,grumpyjames/buck,dsyang/buck,shs96c/buck,neonichu/buck,dsyang/buck,tgummerer/buck,shybovycha/buck,brettwooldridge/buck,mogers/buck,1yvT0s/buck,LegNeato/buck,mogers/buck,dushmis/buck,grumpyjames/buck,vschs007/buck,zpao/buck,robbertvanginkel/buck,liuyang-li/buck,rowillia/buck,clonetwin26/buck,rowillia/buck,darkforestzero/buck,rmaz/buck,marcinkwiatkowski/buck,tgummerer/buck,shybovycha/buck,pwz3n0/buck,luiseduardohdbackup/buck,Dominator008/buck,brettwooldridge/buck,tgummerer/buck,zhuxiaohao/buck,shybovycha/buck,shs96c/buck,shs96c/buck,Addepar/buck,zpao/buck,SeleniumHQ/buck,robbertvanginkel/buck,clonetwin26/buck,mread/buck,mikekap/buck,davido/buck,sdwilsh/buck,k21/buck,OkBuilds/buck,justinmuller/buck,luiseduardohdbackup/buck,dsyang/buck,bocon13/buck,hgl888/buck,1yvT0s/buck,kageiit/buck,JoelMarcey/buck,shybovycha/buck,JoelMarcey/buck,Addepar/buck,neonichu/buck,pwz3n0/buck,shs96c/buck,vine/buck,romanoid/buck,romanoid/buck,tgummerer/buck,JoelMarcey/buck,vine/buck,justinmuller/buck,shybovycha/buck,sdwilsh/buck,darkforestzero/buck,Distrotech/buck,rmaz/buck,brettwooldridge/buck,luiseduardohdbackup/buck,lukw00/buck,zhuxiaohao/buck,dpursehouse/buck,daedric/buck,Learn-Android-app/buck,Dominator008/buck,LegNeato/buck,LegNeato/buck,vine/buck,JoelMarcey/buck,grumpyjames/buck,bocon13/buck,clonetwin26/buck,grumpyjames/buck,ilya-klyuchnikov/buck,liuyang-li/buck,k21/buck,1yvT0s/buck,dpursehouse/buck,raviagarwal7/buck,daedric/buck,Learn-Android-app/buck,illicitonion/buck,marcinkwiatkowski/buck