[Concurrency] Diagnose non-Sendable 'self' arguments crossing actor isolation boundaries in member reference expressions.

[hborla]

Otherwise, the following code bypasses Sendable checking:

class NotSendable {
  @MainActor
  var x: Int { 0 }
}

func test() async {
  let notSendable = NotSendable()
  await notSendable.x // Sending an instance of 'NotSendable' to the MainActor here!
}

Resolves rdar://111453371

[hborla]

@swift-ci please smoke test

[ktoso]

That looks good!

[hborla]

@swift-ci please smoke test