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).