Fix build break when running GenFacades on the WCF projects by partially porting some of the slash changes introduced by recent updates to corefx and buildtools.
Fix build break when running GenFacades on the WCF projects by partially porting some of the slash changes introduced by recent updates to corefx and buildtools.

[tfs-changeset: 1570315]

hongdai/wcf,StephenBonikowsky/wcf,zhenlan/wcf,mconnew/wcf,mconnew/wcf,khdang/wcf,imcarolwang/wcf,dotnet/wcf,iamjasonp/wcf,MattGal/wcf,khdang/wcf,mconnew/wcf,hongdai/wcf,zhenlan/wcf,imcarolwang/wcf,MattGal/wcf,shmao/wcf,dotnet/wcf,KKhurin/wcf,ericstj/wcf,dotnet/wcf,ElJerry/wcf,iamjasonp/wcf,StephenBonikowsky/wcf,ericstj/wcf,imcarolwang/wcf,shmao/wcf,ElJerry/wcf,KKhurin/wcf