[Distributed] After all, a private func may be distributed

[ktoso]

Turns out they can be distributed after all actually.

Because:

distributed actor X { 

  nonisolated func x() async throws {
    try await self.dist() 
  } 

  distributed private func dist() {}
}

[ktoso]

@swift-ci please smoke test and merge

[xedin]

@ktoso could you please add a test-case from the description to the suite too? :)