Use the new lit.util.isMacOSTriple function to determine if a target triple is a macOS target triple.

See 01436117652b41eb389ddbad9c9fd8df0de9b4b2 in llvm.

rdar://26780128
(cherry picked from commit 466f3ab46e3bd128c8cf770c1a7ba729716b99af)
2 files changed