[Support] sys::getProcessTriple should return a macOS triple using

the system's version of macOS

sys::getProcessTriple returns LLVM_HOST_TRIPLE, whose system version might not
be the actual version of the system on which the compiler running. This commit
ensures that, for macOS, sys::getProcessTriple returns a triple with the
system's macOS version.

rdar://33177551

Differential Revision: https://reviews.llvm.org/D34446

llvm-svn: 307372