Bug: monomorphized items have incorrect generics

[Nadrieril]

I discovered in https://github.com/AeneasVerif/charon/pull/723 that we used to not check all items in check_generics, specifically not items created after translation. This revealed that monomorphized items have incorrect generics wrt lifetimes.

I probably broke that myself because I tweaked that recently. Either way, opening that so I don't forget (in https://github.com/AeneasVerif/charon/pull/723 I added a hack so that check_generics keeps not checking monomorphized items x)).

cc @N1ark

[Nadrieril]

The old monomorphization pass has been removed. The new one does also have some generics bugs, from the looks of it just a detail in BuiltinOrAuto most likely coming from on oversight on the hax side.

To close this issue we should remove the is_monomorphized() special case in the check_generics pass.