Don't clear exported_preprocessor_flags.
Don't clear exported_preprocessor_flags.

Summary:
We set exported_preprocessor_flags in the populated args but then
clear them again. This fixes that.

Test Plan: CI

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