Auto merge of rust-lang#101679 - compiler-errors:rpitit-default-body, r=nikomatsakis

Support default-body trait functions with return-position `impl Trait` in traits

Introduce a new `Trait` candidate kind for the `ImplTraitInTrait` projection candidate, which just projects an RPITIT down to its opaque type form.

This is a hack until we lower RPITITs to regular associated types, after which we will need to rework how these default bodies are type-checked, so comments are left in a few places for us to clean up later.

Fixes rust-lang#101665

Author: bors