Rollup merge of #132194 - compiler-errors:rpitit-super-wc, r=spastorino

Collect item bounds for RPITITs from trait where clauses just like associated types

We collect item bounds from trait where clauses for *associated types*, i.e. this:

```rust
trait Foo
where
    Self::Assoc: Send
{
    type Assoc;
}
```

Becomes this:

```rust
trait Foo {
    type Assoc: Send;
}
```

Today, with RPITITs/AFIT and return-type notation, we don't do that, i.e.:

```rust
trait Foo where Self::method(..): Send {
    fn method() -> impl Sized;
}

fn is_send(_: impl Send) {}
fn test<T: Foo>() {
    is_send(T::method());
}
```

...which fails on nightly today.

 Turns out it's super easy to fix this, and we just need to use the `associated_type_bounds` lowering function in `explicit_item_bounds_with_filter`, which has that logic baked in.

Author: workingjubilee