[Concurrency] Diagnose non-sendable 'self' arguments crossing isolation boundaries.

[hborla]

Otherwise, the following code can reference the state of a non-Sendable class across different actor isolation domains:

final class NonSendable {
  var value = ""

  @MainActor
  func update() {
    value = "update"
  }
}

func test() async {
  let t = NonSendable()
  await t.update() // Sending an instance of 'NonSendable' to the MainActor here!
}

Resolves rdar://111470341

[hborla]

@swift-ci please smoke test

[ktoso]

Great catch, I think that's the right thing to do 👍

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test macOS