folly/ExceptionWrapper.h: avoid warning about typeid operand side effects
folly/ExceptionWrapper.h: avoid warning about typeid operand side effects

Summary:
* folly/ExceptionWrapper.h (class_name): Clang is not yet smart enough to
see that there is no harm in dereferencing an "item_" that we've just
verified is non-NULL.  Accommodate it, to avoid this:
./folly/ExceptionWrapper.h:199:30: error: expression with side effects will be evaluated despite being used as an operand to 'typeid' [-Werror,-Wpotentially-evaluated-expression]
return demangle(typeid(*item_));
^

Test Plan:
Run these commands and note there are fewer errors than before:
fbconfig --clang --with-project-version=clang:dev -r folly && fbmake dbgo

Reviewed By: mhorowitz@fb.com

Subscribers: trunkagent, yfeldblum, folly-diffs@

FB internal diff: D1848327

Tasks: 6244745

Signature: t1:1848327:1423869597:a58c9e9e3671befb78ae07fbd274a13d08ffb2a7

Blame Revision:

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