Fix cycle error with existential types

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:

```
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')