stdlib: enable prespecialization of generic metadata on Windows
stdlib: enable prespecialization of generic metadata on Windows

Enable support for pre-speciailization of generic metadata on Windows.

JGiola/swift,benlangmuir/swift,gregomni/swift,roambotics/swift,ahoppen/swift,apple/swift,rudkx/swift,atrick/swift,roambotics/swift,atrick/swift,roambotics/swift,roambotics/swift,apple/swift,JGiola/swift,atrick/swift,gregomni/swift,atrick/swift,atrick/swift,gregomni/swift,ahoppen/swift,JGiola/swift,apple/swift,benlangmuir/swift,glessard/swift,benlangmuir/swift,roambotics/swift,glessard/swift,apple/swift,glessard/swift,rudkx/swift,benlangmuir/swift,rudkx/swift,gregomni/swift,glessard/swift,glessard/swift,apple/swift,gregomni/swift,JGiola/swift,gregomni/swift,rudkx/swift,apple/swift,rudkx/swift,glessard/swift,rudkx/swift,roambotics/swift,ahoppen/swift,ahoppen/swift,ahoppen/swift,ahoppen/swift,JGiola/swift,atrick/swift,benlangmuir/swift,JGiola/swift,benlangmuir/swift