Auto merge of #94911 - jackh726:gats_extended_2, r=compiler-errors

Auto merge of #94911 - jackh726:gats_extended_2, r=compiler-errors

Make GATs object safe under generic_associated_types_extended feature

Based on #94869

Let's say we have
```rust
trait StreamingIterator {
    type Item<'a> where Self: 'a;
}
```
And `dyn for<'a> StreamingIterator<Item<'a> = &'a i32>`.

If we ask `(dyn for<'a> StreamingIterator<Item<'a> = &'a i32>): StreamingIterator`, then we have to prove that `for<'x> (&'x i32): Sized`. So, we generate *new* bound vars to subst for the GAT generics.

Importantly, this doesn't fully verify that these are usable and sound.

r? `@nikomatsakis`