Convert ArchSpec::ParseMachOCPUDashSubtypeTriple to use StringRef.

This makes the code easier to grok, and since this is a very low
level function it also is very helpful to have this take a StringRef
since it means anyone higher up the chain who has a StringRef would
have to first convert it to a null-terminated string.  This way it
can work equally well with StringRefs or const char*'s, which will
enable the conversion of higher up functions to StringRef.

Tested on Windows, Linux, and OSX and saw no regressions.

llvm-svn: 281642