[Driver] Add a "-mmacos_version_min" option that's an alias for

"-mmacosx_version_min"

The option -mmacosx_version_min will still be the canonical option for now, but
in the future we will switch over to -mmacos_version_min and make
-mmacosx_version_min an alias instead.

rdar://27043820

Differential Revision: https://reviews.llvm.org/D32796

llvm-svn: 302240