[clang][driver] add clang driver support for emitting macho files with two...

[clang][driver] add clang driver support for emitting macho files with two build version load commands

This patch extends clang driver to pass the right flags to the clang frontend, and ld64,
so that they can emit macho files with two build version load commands. It adds a new
0darwin-target-variant option which complements -target and also can be used to specify different
target variants when multi-arch compilations are invoked with multiple -arch commands.

Differential Revision: https://reviews.llvm.org/D118862