Skip inner obligations of ImplSource::Builtin

[Nadrieril]

As they can lead to infinite recursion on e.g. this:

fn builtin_bound<M: Send>(_msg: M) {}
fn use_builtin_bound() {
    builtin_bound(());
}

I tried to add a test in your test suite but there was no bug on hax itself; it seems this was only triggered in charon.

[W95Psp]

Looks good to me, thanks!