[driver][darwin] Take the OS version from -m<os>-version-min argument when

-target has no OS version

This ensures that Clang won't warn about redundant -m<os>-version-min
argument for an invocation like
`-target x86_64-apple-macos -mmacos-version-min=10.11`


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@321559 91177308-0d34-0410-b5e6-96231b3b80d8