Auto merge of #146329 - lcnr:opaque-type-infer-alias-candidates, r=BoxyUwU

consider item bounds for non-yet-defined opaque types

Based on rust-lang/rust#140405.

fixes rust-lang/trait-system-refactor-initiative#182
fixes rust-lang/trait-system-refactor-initiative#196
fixes rust-lang/trait-system-refactor-initiative#205

there's some jank here, see rust-lang/trait-system-refactor-initiative#229

## Design

If the self type is an inference variable which has been sub-unified with am opaque type, we need to incompletely guide inference to avoid breakage.

In this case, we
- look at the item bounds of all sub-unified opaque types, and
- blanket impls which do not constrain the self type

Even if there are applicable candidates, we always force their certainty to be `Maybe`, so they will always have to be reproven once we've constrained the inference variable.

This is a bit iffy, see the added tests.

r? `@BoxyUwU`

Author: bors