AvailabilityAttr: we accept "macos" as the platform name.

We continue accepting "macosx" but canonicalize it to "macos", When emitting
diagnostics, we use "macOS" instead of "OS X".

The PlatformName in TargetInfo is changed from "macosx" to "macos" so we can
directly compare the Platform in AvailabilityAttr with the PlatformName
in TargetInfo.

rdar://26795172
rdar://26800775

llvm-svn: 274064