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

In presence of constructor elision, the contract of f didn't make sense

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.

Actually, they appear only in the AST generated by clang-11.

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.