With this patch -Wwrite-strings is still implemented with the terrible

With this patch -Wwrite-strings is still implemented with the terrible
hack of passing -fconst-strings to -cc1, but at least the driver uses
the regular warning checking code path.

Since we don't support a warning that is DefaultIgnore in one language
but not in another, this patch creates a dummy C only warning in the same
group as the existing one to get the desired effect.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194097 91177308-0d34-0410-b5e6-96231b3b80d8