Use StringRef instead of returning a temporary std::string.

This fixes a really nasty bug in Darwin::getDarwinArchName where we were going
StringRef -> temporary std::string -> StringRef (and return the dead StringRef).
The StringRefs from Triple live as long as the Triple itself, that should be
long enough.

Hopefully 2 of 4 MSVC buildbot failures are gone now.

llvm-svn: 94892