Add supertrait item shadowing for type-level path resolution

[Amanieu]

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 #148605 about the inconsistency with name resolution in expressions and method calls.

[rustbot]

HIR ty lowering was modified

cc @fmease

[rustbot]

r? @madsmtm

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[jackh726]

What about T: B<T = i16> and T: A<T = i8> - do those resolve to B::T and A::T respectively?

Also, please use a different name for the generic and the associated type, it's a bit confusing as-is.

[Amanieu]

I've cleaned up the example and added tests showing how B<T = i16> and A<T = i8> are resolved.

[jackh726]

r? types

[madsmtm]

r? types

Wait, I'm not sure if that should've unassigned me or not? In any case, if you also need a compiler reviewer, I'm the wrong one for the job, so I'm gonna unassign myself.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.