Fix ICE #152158: RTN through trait alias

[rynewang]

Summary

Fix ICE when using return type notation (RTN) through a trait alias.

Problem

#![feature(return_type_notation, trait_alias)]
trait Tr { fn f() -> impl Sized; }
trait Al = Tr;
fn f<T: Al<f(..): Copy>>() {}

ICEs with span_delayed_bug("bad return type notation here").

Root Cause

Two places in resolve_bound_vars.rs only check for DefKind::Trait but not DefKind::TraitAlias:

1. The type_def_id resolution match
2. The RTN guard condition

Fix

Add DefKind::TraitAlias to both locations. supertrait_hrtb_vars() already handles trait aliases.

Includes regression test.

Fixes rust-lang#152158