Switch around the order of MSVCCompat and MicrosoftExt

Full language modes usually get listed before minor language extensions in
LangOpts, so that subsequent sub-modes can predicate on the major modes.

This also lends to a cleanup in CompilerInvocation to better indicate to the
reader that MSVCCompat is a superset of MicrosoftExt.

Cleanup only.

llvm-svn: 199210