Rollup merge of #152225 - Amanieu:type-supertrait-shadowing, r=jackh726

Add supertrait item shadowing for type-level path resolution

This makes type-level name resolution (used primarily for associated types) also prefer subtrait items to supertrait items in case of ambiguity.

This addresses the concern from rust-lang/rust#148605 about the inconsistency with name resolution in expressions and method calls.

Author: jhpratt