Auto merge of #54262 - matthewjasper:explain-in-typeck, r=nikomatsakis

[NLL] Record more infomation on free region constraints in typeck

Changes:

* Makes the span of the MIR return place point to the return type
* Don't try to use a path to a type alias as a path to the adt it aliases (fixes an ICE)
* Don't claim that `self` is declared outside of the function. [see this test](https://github.com/rust-lang/rust/commit/f2995d5b1ad0401a80ac4579ebc89295818c3eed#diff-0c9e6b1b204f42129b481df9ce459d44)
* Remove boring/interesting distinction and instead add a `ConstraintCategory` to the constraint.
* Add categories for implicit `Sized` and `Copy` requirements, for closure bounds, for user type annotations and `impl Trait`.
* Don't use the span of the first statement for Locations::All bounds (even if it happens to work on the tests we have)

Future work:

* Fine tuning the heuristic used to choose the place the report the error.
* Reporting multiple places (behind a flag)
* Better closure bounds reporting. This probably requires some discussion.

r? @nikomatsakis

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