Support -Wsign-compare compilation
Support -Wsign-compare compilation

Summary:
Due to how Conv.h uses the less_than template inside
not-actually-static static ifs clang still ends up validating generated
code inside a FOLLY_RANGE_CHECK which is never executed due to the
templated types. This code however still generates -Wsign-compare issues
so suppress that in order to allow includers to use this flag.

A simple example will illustrate this:

uint64_t foo = 1;
int bar = folly::to<int>(foo);

Test Plan: fbmake runtests

Reviewed By: meyering@fb.com, njormrod@fb.com

Subscribers: trunkagent, folly-diffs@, bmatheny, ranjeeth, subodh, kmdent, fma, shikong, pgriess, jdelong

FB internal diff: D1731411

Signature: t1:1731411:1418243200:ed1f34a1485669c9cb18f9f6029aca70e498953c

kernelim/folly,theiver9827/folly,gaoyingie/folly,bikong2/folly,clearlinux/folly,tomhughes/folly,romange/folly,loversInJapan/folly,stonegithubs/folly,Hincoin/folly,bsampath/folly,constantine001/folly,xzmagic/folly,mqeizi/folly,yangjin-unique/folly,guker/folly,SeanRBurton/folly,SeanRBurton/folly,xzmagic/folly,bowlofstew/folly,yangjin-unique/folly,fw1121/folly,lifei/folly,brunomorishita/folly,gaoyingie/folly,raphaelamorim/folly,CJstar/folly,theiver9827/folly,shaobz/folly,sonnyhu/folly,theiver9827/folly,clearlylin/folly,bikong2/folly,cppfool/folly,tempbottle/folly,Eagle-X/folly,mqeizi/folly,cppfool/folly,brunomorishita/folly,tempbottle/folly,sonnyhu/folly,bowlofstew/folly,alexst07/folly,chjp2046/folly,hongliangzhao/folly,yangjin-unique/folly,clearlinux/folly,renyinew/folly,stonegithubs/folly,doctaweeks/folly,zhiweicai/folly,loversInJapan/folly,juniway/folly,bobegir/folly,SeanRBurton/folly,gavioto/folly,mqeizi/folly,fw1121/folly,brunomorishita/folly,CJstar/folly,arg0/folly,bowlofstew/folly,gaoyingie/folly,alexst07/folly,Hincoin/folly,yangjin-unique/folly,charsyam/folly,facebook/folly,CJstar/folly,bsampath/folly,gavioto/folly,kernelim/folly,lifei/folly,xzmagic/folly,fw1121/folly,stonegithubs/folly,cole14/folly,leolujuyi/folly,tomhughes/folly,SammyK/folly,kernelim/folly,juniway/folly,PPC64/folly,sonnyhu/folly,SammyK/folly,wildinto/folly,clearlylin/folly,guker/folly,sakishum/folly,raphaelamorim/folly,tomhughes/folly,project-zerus/folly,loverszhaokai/folly,alexst07/folly,theiver9827/folly,loversInJapan/folly,constantine001/folly,Orvid/folly,kernelim/folly,chjp2046/folly,project-zerus/folly,brunomorishita/folly,CJstar/folly,tempbottle/folly,raphaelamorim/folly,cole14/folly,ykarlsbrun/folly,sonnyhu/folly,hongliangzhao/folly,Eagle-X/folly,loverszhaokai/folly,PoisonBOx/folly,stonegithubs/folly,juniway/folly,clearlinux/folly,guker/folly,shaobz/folly,juniway/folly,renyinew/folly,doctaweeks/folly,bikong2/folly,gavioto/folly,bowlofstew/folly,PoisonBOx/folly,upsoft/folly,floxard/folly,rklabs/folly,KSreeHarsha/folly,PoisonBOx/folly,floxard/folly,Orvid/folly,SammyK/folly,clearlinux/folly,sakishum/folly,bikong2/folly,romange/folly,wildinto/folly,Eagle-X/folly,Hincoin/folly,ykarlsbrun/folly,reddit/folly,nickhen/folly,mqeizi/folly,fw1121/folly,tempbottle/folly,ykarlsbrun/folly,constantine001/folly,colemancda/folly,constantine001/folly,lifei/folly,charsyam/folly,Eagle-X/folly,CJstar/folly,nickhen/folly,upsoft/folly,sonnyhu/folly,KSreeHarsha/folly,upsoft/folly,rklabs/folly,colemancda/folly,sakishum/folly,leolujuyi/folly,bobegir/folly,leolujuyi/folly,xzmagic/folly,wildinto/folly,xzmagic/folly,hongliangzhao/folly,zhiweicai/folly,loverszhaokai/folly,rklabs/folly,nickhen/folly,shaobz/folly,charsyam/folly,floxard/folly,PoisonBOx/folly,Hincoin/folly,guker/folly,romange/folly,Orvid/folly,PPC64/folly,tomhughes/folly,project-zerus/folly,tempbottle/folly,juniway/folly,bobegir/folly,reddit/folly,sakishum/folly,shaobz/folly,PPC64/folly,leolujuyi/folly,leolujuyi/folly,hongliangzhao/folly,cppfool/folly,SammyK/folly,bsampath/folly,renyinew/folly,zhiweicai/folly,SeanRBurton/folly,colemancda/folly,shaobz/folly,bsampath/folly,clearlylin/folly,romange/folly,arg0/folly,alexst07/folly,wildinto/folly,cole14/folly,bsampath/folly,ykarlsbrun/folly,doctaweeks/folly,brunomorishita/folly,mqeizi/folly,Eagle-X/folly,sakishum/folly,project-zerus/folly,arg0/folly,lifei/folly,gavioto/folly,nickhen/folly,tomhughes/folly,lifei/folly,loversInJapan/folly,zhiweicai/folly,alexst07/folly,cppfool/folly,facebook/folly,loversInJapan/folly,romange/folly,KSreeHarsha/folly,KSreeHarsha/folly,chjp2046/folly,floxard/folly,yangjin-unique/folly,facebook/folly,reddit/folly,SammyK/folly,nickhen/folly,cole14/folly,reddit/folly,Orvid/folly,fw1121/folly,renyinew/folly,doctaweeks/folly,renyinew/folly,bikong2/folly,raphaelamorim/folly,clearlylin/folly,cole14/folly,arg0/folly,Orvid/folly,facebook/folly,gavioto/folly,raphaelamorim/folly,hongliangzhao/folly,bowlofstew/folly,PoisonBOx/folly,stonegithubs/folly,kernelim/folly,loverszhaokai/folly,gaoyingie/folly,upsoft/folly,PPC64/folly,floxard/folly,clearlinux/folly,chjp2046/folly,arg0/folly,zhiweicai/folly,reddit/folly,facebook/folly,rklabs/folly,charsyam/folly,ykarlsbrun/folly,PPC64/folly,gaoyingie/folly,SeanRBurton/folly,colemancda/folly,clearlylin/folly,loverszhaokai/folly,colemancda/folly,KSreeHarsha/folly,doctaweeks/folly,guker/folly,wildinto/folly,theiver9827/folly,constantine001/folly,upsoft/folly,bobegir/folly,cppfool/folly,chjp2046/folly,Hincoin/folly,bobegir/folly,rklabs/folly,project-zerus/folly