clang-cl: Fall back immediately if /GR and /fallback were both passed

None of our tests use /fallback, so this lets us gradually add RTTI
support without breaking projects using /fallback.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208787 91177308-0d34-0410-b5e6-96231b3b80d8