Ensure RPITITs are created before def-id freezing
From the test:
```rust
// `ty::Error` in a trait ref will silence any missing item errors, but will also
// prevent the `associated_items` query from being called before def ids are frozen.
```
Essentially, the code that checks that `impl`s have all their items (`check_impl_items_against_trait`) is also (implicitly) responsible for fetching the `associated_items` query before, but since we early return here:
https://github.com/rust-lang/rust/blob/c2901f543577af99b9cb708f5c0d28525eb7f08f/compiler/rustc_hir_analysis/src/check/check.rs#L732-L737
...that means that this never happens for trait refs that reference errors.
Fixes#122518
r? oli-obk
Matthias Krüger
ee940f87fc