hide -kernel-msg-key behind a proper FCLang option

This is arguably a short-term fix, and a better fix would be to rely
directly on big integers, at least on OCaml side, in the intermediate format.

so that they can be defined in one translation unit while the class is
declared multiple times.

especially if half of them are not seen as implicit afterwards. This can't end
well 

it seems that this is not consistent across clang versions.