clang-cl: Warn about overriding /MD with /MT etc.

This also bakes the /M options into a separate option group to make
them easier to refer to from the code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190529 91177308-0d34-0410-b5e6-96231b3b80d8