[region-isolation] When determining isolation from a class_method, check for global actor isolation first.

[gottesmm]

Before this change in the following code, we would say that message is isolated to the actor instead of the global actor isolation of the actor's method:

class Message { ... }

actor MessageHolder {
  @MainActor func hold(_ message: Message) { ... }
}

@MainActor
func sendMessage() async {
    let messageHolder = MessageHolder()
    let message = Message()
    // We identified messageHolder.hold as being MessageHolder isolated
    // instead of main actor isolated.
    messageHolder.hold(message)
    Task { @MainActor in print(message) }
}

I also used this as an opportunity to simplify the logic in this part of the code. Specifically, I had made it so that multiple interesting cases were handled in the same conditional statement in a manner that it made it hard to know which cases were actually being handled and why it was correct. Now I split that into two separate if statements with comments that make it clear what we are actually trying to pattern match against.

rdar://130980933

[gottesmm]

@swift-ci smoke test

[ktoso]

Late LGTM, looks ok