Add missing OS check to r305678

That commit failed on non-macOS buildbots as I've forgotten to make sure that
the system on which Clang is running on is actually macOS.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305680 91177308-0d34-0410-b5e6-96231b3b80d8