[patch] Slightly less hackish implementation of -Wwrite-strings

[Rafael Espíndola]

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.

Chandler, is the include of clang/Sema/SemaDiagnostic.h OK in here or
some extra refactoring is needed?

Cheers,
Rafael
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