folly/stats - fix c++ spec compliance for call to template name
folly/stats - fix c++ spec compliance for call to template name

Summary:
This code is not strictly correct per c++ spec section 14.2.4:

> When the name of a member template specialization appears after . or -> in a
> postfix-expression or after a nested-name-specifier in a qualified-id, and the
> object or pointer expression of the postfix-expression or the
> nested-name-specifier in the qualified-id depends on a template parameter
> (14.6.2) but does not refer to a member of the current instantiation
> (14.6.2.1), the member template name must be prefixed by the keyword template.
> Otherwise the name is assumed to name a non-template.

This causes clang to complain, but gcc is lax about this, as usual.

Test Plan: compile stuff that use this.

Reviewed By: delong.j@fb.com

FB internal diff: D643515

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