Auto merge of #54453 - nikomatsakis:nll-issue-53121-shred-outlives, r=pnkfelix

rework how we handle outlives relationships

When we encounter an outlives relationship involving a projection, we use to over-constrain in some cases with region constraints. We also used to evaluate whether the where-clauses in the environment might apply **before** running inference.

We now avoid doing both of those things:

- If there are where-clauses in the environment that might be useful, we add no constraints.
- After inference is done, we check if we wound up inferring values compatible with the where-clause, and make use of them if so.

I realize now that this PR includes some meandering commits and refactorings from when I expected to go in a different direction. If desired, I could try to remove some of those.

Fixes #53121
Fixes #53789

r? @pnkfelix

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