[Later][Distributed] allow extensions where Self: DA, but the extended protocol is not DA explicitly

[ktoso]

rdar://91857262

This is actually pretty tricky so leaving it for now but will get back to it.

The issue is about:

protocol GamePlayer: Sendable, Identifiable where ID == FakeActorSystem.ActorID {
...
}
extension GamePlayer where Self: DistributedActor, ActorSystem == FakeActorSystem {
  distributed func startGame(with opponent: String) -> String { ... }
}

distributed actor FishPlayer: GamePlayer {
  typealias ActorSystem = FakeActorSystem
}

so we should be able to call startGame on the FishPlayer -- it is guaranteed to be right; but in order to synthesize the thunk for the startGame in the extension (!) we need to call to the protocol's actorSystem property, which we don't handle well today.

Assertion failed: (systemProperty && "Unable to find 'actorSystem' property"), function deriveBodyDistributed_thunk, file CodeSynthesisDistributedActor.cpp, line 300.

Some attempts in PR but this is incomplete, only showing the issue.

[ktoso]

This works now, and test here confirms it: #68451