Rollup merge of #92917 - jackh726:issue-91762-2, r=nikomatsakis

Rollup merge of #92917 - jackh726:issue-91762-2, r=nikomatsakis

Don't constrain projection predicates with inference vars in GAT substs

cc #91762

Not a fix, but a mitigation to prevent a backwards-compatible hazard where we normalize using a predicate only because it's the only one available, but shouldn't. This would constrain an inference variable which didn't really want. We already do this when selecting a projection candidate, which isn't always correct. But changing that is a problem for a different day.

Also found out that a suggestion for `await`ing a future was using the wrong substs.

r? ``@nikomatsakis``