Clang: Implicitly define identifying macro when using clang code model

Clang's diagnostic pragmas work with the clang code model but there is no identifying macro left to guard such pragmas since _clang_ (and related) default macros are explicitly undefined.

It would be nice if there were an alternative macro, implicitly defined by QtCreator when using the clang code model, that could be used instead.

(Similar to how Q_CREATOR_RUN is provided for the builtin code model)