Also expand weak alias tys inside consts inside `expand_weak_alias_tys`
Ever since #121344 has been merged, I couldn't let go of the fear that I might've slipped a tiny bug into rustc (:P).
Checking the type flags of the `Const` is strictly more correct than only checking the ones of the `Const`'s `Ty`. I don't think it's possible to trigger an ICE rn (i.e., one of the two `bug!("unexpected weak alias type")` I added in branches where `expand_weak_alias_tys` should've expanded *all* weak alias tys) because presently const exprs aren't allowed to capture late-bound vars. To be future-proof however, we should iron this out.
A possible reproducer would be the following if I'm not mistaken (currently fails to compile due to the aforementioned restriction):
```rs
#![feature(lazy_type_alias, adt_const_params, generic_const_exprs)]
type F = for<'a> fn(A<{ S::<Weak<'a>>(loop {}) }>) -> &'a ();
type A<const N: S<Weak<'static>>> = ();
#[derive(PartialEq, Eq, std::marker::ConstParamTy)]
struct S<T>(T);
type Weak<'a> = &'a ();
```
Whether a late-bound region should actually be considered constrained by a const expr is a separate question — one which we don't need to answer until / unless we actually allow them in such contexts (probable answer: only inside the return exprs of a block but not inside the stmts).
r? oli-obk (he's not available rn but that's fine) or types or compiler
Matthias Krüger
03ff673dcc