Rollup merge of rust-lang#102338 - compiler-errors:assoc-ty-binding-i…

…n-assoc-ty-binding, r=cjgillot

Deny associated type bindings within associated type bindings

Fixes rust-lang#102335

This was made worse by rust-lang#100865, which unified the way we generate substs for GATs and non-generic associated types. However, the issue was not _caused_ by rust-lang#100865, evidenced by the test I added for GATs:

```rust
trait T {
    type A: S<C<(), i32 = ()> = ()>;
    //~^ ERROR associated type bindings are not allowed here
}

trait Q {}

trait S {
    type C<T>: Q;
}

fn main() {}
```

^ which passes on beta (where GATs are stable) and presumably ever since GATs support was added to `create_substs_for_associated_item` in astconv.