[Support] Accept macosx triple as 'darwin' in Host unittest. NFC.
[Support] Accept macosx triple as 'darwin' in Host unittest. NFC.

If LLVM was configured with an x86_64-apple-macosx host triple, this
test would fail, as the API works but the triple isn't in the whitelist.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@293990 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm