- Drop `mayHaveSuperclass` because it's too restrictive.
- Add logic to get superclass of existential and re-create
existential type with all of the protocol requirements.
If the underlying key path is not Sendable, the compiler generated
closure that captures the key path expression (as `{ [$kp$ = ...] in $0[keyPath: $kp$] }`)
cannot be marked as Sendable either.
Allow to compose key path type with a protocol as a way to express
additional requirements on the parameter. For example:
```
struct Test<R> {
subscript<V>(dynamicMember member: KeyPath<R, V> & Sendable) -> V {
...
}
}
```
