Deny associated type bindings within associated type bindings
Fixes#102335
This was made worse by #100865, which unified the way we generate substs for GATs and non-generic associated types. However, the issue was not _caused_ by #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.
