When other errors are generated, there can be a mismatch between the
amount of input types in MIR, and the amount in the function itself.
Break from the comparative loop if this is the case to prevent
out-of-bounds.

A `Body` now contains its `MirSource`, which in turn contains the
`DefId` of the item associated with the `Body`.

This handles the case where a user explicitly annotations a closure
signature with a opaque return type.

Fixes #63263

The commit should have changed comments as well.
At the time of writting, it passes the tidy and check tool.

Revisions asked by eddyb :
- Renamed of all the occurences of {visit/super}_mir
- Renamed test structures `CachedMir` to `Cached`

Fixing the missing import on `AggregateKind`

Allows us to use the category of outlive requirements inside a closure
when reporting free region errors caused by its closure bounds.