Auto merge of #80828 - SNCPlay42:opaque-projections, r=estebank

Auto merge of #80828 - SNCPlay42:opaque-projections, r=estebank

Fix expected/found order on impl trait projection mismatch error

fixes #68561

This PR adds a new `ObligationCauseCode` used when checking the concrete type of an impl trait satisfies its bounds, and checks for that cause code in the existing test to see if a projection's normalized type should be the "expected" or "found" type.

The second commit adds a `peel_derives` to that test, which appears to be necessary in some cases (see projection-mismatch-in-impl-where-clause.rs, which would still give expected/found in the wrong order otherwise). This caused some other changes in diagnostics not involving impl trait, but they look correct to me.