Rollup merge of #62423 - Aaron1011:fix/existential-cycle, r=oli-obk

Rollup merge of #62423 - Aaron1011:fix/existential-cycle, r=oli-obk

Fix cycle error with existential types

Fixes #61863

We now allow uses of `existential type`'s that aren't defining uses - that is, uses which don't constrain the underlying concrete type.

To make this work correctly, we also modify `eq_opaque_type_and_type` to not try to apply additional constraints to an opaque type. If we have code like this:

```rust
existential type Foo;
fn foo1() -> Foo { ... }
fn foo2() -> Foo { foo1() }
```

then `foo2` doesn't end up constraining `Foo`, which means that `foo2` will end up using the type `Foo` internally - that is, an actual `TyKind::Opaque`. We don't want to equate this to the underlying concrete type - we just need to enforce the basic equality constraint between the two types (here, the return type of `foo1` and the return type of `foo2`)