Require impl Trait in associated types to appear in method signatures

[oli-obk]

This implements the limited version of TAIT that was proposed in #107645 (comment)

Similar to impl Trait in return types, impl Trait in associated types may only be used within the impl block which it is a part of. To make everything simpler and forward compatible to getting desugared to a plain type alias impl trait in the future, we're requiring that any associated functions or constants that want to register hidden types must be using the associated type in their signature (type of the constant or argument/return type of the associated method. Where bounds mentioning the associated type are ignored).

We have preexisting tests checking that this works transitively across multiple associated types in situations like

impl Foo for Bar {
    type A = impl Trait;
    type B = impl Iterator<Item = Self::A>;
    fn foo() -> Self::B { ...... }
}