Add some unit tests for ArchSpec.

I'm was trying to do some cleanup and code modernization and in
doing so I needed to change ParseMachCPUDashSubtypeTriple to take
a StringRef.  To ensure I don't break anything, I'm adding some
unit tests for this function.  As a side benefit, this also expands
test coverage of this function to all platforms, since in general
this code would rarely be exercised on non Mac platforms, and never
in the test suite.

llvm-svn: 281387