Rollup merge of #118914 - compiler-errors:eager-alias-relate, r=lcnr

Unconditionally register alias-relate in projection goal

Follow-up to #118725, which subtly broke closure signature inference on combinators like `Result::map` which I noticed in syn.

Essentially, instead of using `eq` which will eagerly infer `?1 := <?2 as Trait>::Assoc`, we can directly emit an alias-relate goal, which will stay ambiguous for as long as `?2` is ambiguous.

This also more closely models the conceptual framing that projects-to acts like an alias-relate when solving, and like a normalizes-to when in a param env.

r? lcnr