Driver: correct behaviour of -fmsc-version=MAJOR

Ensure that we properly handle the case where just the major version component
is provided by the user.

Thanks to Alp Toker for pointing out that this was not handled correctly!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211506 91177308-0d34-0410-b5e6-96231b3b80d8