wangle: make new pragma use portable to older clang
wangle: make new pragma use portable to older clang

Summary:
* folly/wangle/channel/test/MockChannelHandler.h:
Avoiding a new warning that rendered this file uncompilable
by clang-3.5 and older.  Add cpp conditionals to protect
against that.

Test Plan:
Ensure it works with clang-3.4, 3.5 and clang:dev.
I.e., ensure that this prints PASS at the end.

for i in '' 3.5 dev; do
test -n "$i" && i=--with-project-version=clang:$i
fbconfig -r --clang $i folly/wangle/channel/test && fbmake runtests_dbgo
done && echo PASS

Reviewed By: meyering@fb.com

Subscribers: mpawlowski, fugalh, mathieubaudet, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1849474

Tasks: 6244745

Blame Revision: D1848330
@build-break

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