chore: delete unused ui/auxiliary crates
It appears that all the tests that used it have been moved to tests/ui/editions/ (or elsewhere) already.
r? `````@jieyouxu`````
- `tests/ui/augmented-assignment-feature-gate-cross.rs`:
- This was *originally* to feature-gate overloaded OpAssign
cross-crate, but now let's keep it as a smoke test.
- Renamed as `augmented-assignment-cross-crate.rs`.
- Relocated under `tests/ui/binop/`.
- `tests/ui/augmented-assignments.rs`:
- Documented test intent.
- Moved under `tests/ui/borrowck/`.
- `tests/ui/augmented-assignment-rpass.rs`:
- Renamed to drop the `-rpass` suffix, since this was leftover from
when `run-pass` test suite was a thing.
- Moved under `tests/ui/binop/`.
If a type has unsafe fields, its safety invariants are not simply
the conjunction of its field types' safety invariants. Consequently,
it's invalid to reason about the safety properties of these types
in a purely structural manner — i.e., the manner in which `auto`
traits are implemented.
Makes progress towards #132922.
