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.
Dylan DPC
e9f391e09a