Properly use parent generics for opaque types

Properly use parent generics for opaque types

Fixes #67844

Previously, opaque types would only get parent generics if they
a return-position-impl-trait (e.g. `fn foo<A>() -> impl MyTrait<A>`).

However, it's possible for opaque types to be nested inside one another:

```rust
trait WithAssoc { type AssocType; }

trait WithParam<A> {}

type Return<A> = impl WithAssoc<AssocType = impl WithParam<A>>;
```

When this occurs, we need to ensure that the nested opaque types
properly inherit generic parameters from their parent opaque type.

This commit fixes the `generics_of` query to take the parent item
into account when determining the generics for an opaque type.