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.

Mixing package sources can link to interesting linking issues.

It turns out that llvm-11 is now included in nixos-20.03, our main source
in Frama-CI as of now.

Turns out that some warnings are only emitted with `-O1`...

We don't need it, and it is not always enabled in LLVM or clang, without
reliable way to check (`llvm-config --has-rtti` can return `YES` while clang
is built without rtti).

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.

also prepare running tests on LLVM 11.