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

[gottesmm]

Explanation: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) }
}

It was just b/c we were checking the global actor bit on the field after checking for if it is apart of an actor. With this change, we swap the order and I split up/commented the logic in this part of the code to separate different cases that we are handling to make it easier to understand/prevent future errors.

Radars:

• rdar://130980933

Original PRs:

• [region-isolation] When determining isolation from a class_method, check for global actor isolation first. #75016

Risk: Low. This just tweaks the behavior around how SIL isolation info identifies SILIsolationInfo of class methods. It doesn't touch the rest of the compiler... so any potential problems would be localized to class_method making this a targeted fix. Given the simplicity of the change, I do not expect any follow on problems.
Testing: Added/Ran tests
Reviewer: N/A

[gottesmm]

@swift-ci test

[Sajjon]

typo: appllies -> applies

[gottesmm]

I'll fix that on main. This is for a cherry-pick to 6.0.