Allow ~const on associated type bounds again

[fmease]

This follows from this Zulip discussion.

Basically in my opinion, it makes sense to allow ~const on associated type bounds again since they're quite useful even though we haven't implemented the proposed syntax <Ty as ~const Trait>::Proj/<Ty as const Trait>::Proj yet; that can happen as a follow-up.

This already allows more code to compile since T::Assoc where T is a type parameter and where the predicate <T as ~const Trait> is in the environment gets elaborated to (pseudo) <T as ~const Trait>::Assoc.

#[const_trait]
trait Trait {
    type Assoc: ~const Trait;
    fn func() -> i32;
}

const fn function<T: ~const Trait>() -> i32 {
    T::Assoc::func()
}

~const associated type bounds also work together with const bounds:

struct Type<const N: i32>;

fn procedure<T: const Trait>() -> Type<{ T::Assoc::func() }> { // `Trait` comes from above
    Type
}

NB: This PR also starts allowing ~const bounds in the generics and the where-clause of trait associated types since it's trivial to support them. However, I don't know if those bounds are actually useful. Maybe we should continue to reject them?
For reference, it wouldn't make any sense to allow ~const Trait in GACs (generic associated constants, generic_const_items) because they'd be absolutely useless (contrary to const Trait).

[@]rustbot ping project-const-traits
r? project-const-traits

[compiler-errors]

However, I don't know if those bounds are actually useful.

Yes, I think these GAT where bounds are both useful and necessary as long as we all ~const in item bound position. Some associated types may be conditionally ~const only if some generic type is ~const. For example:

#[const_trait] trait Foo {}
impl const Foo for Wrapper<T> where T: ~const Foo {}

#[const_trait] trait Bar {
  type Assoc<T: ~const Foo>: ~const Foo;
}
impl Bar for () {
  type Assoc<T: ~const Foo> = Wrapper<T>; // this is **only** `~const Foo` if `T: ~const Foo`
}

@bors r+

[bors]

📌 Commit f283739 has been approved by compiler-errors

It is now in the queue for this repository.