[6.0][Concurrency] Don't diagnose nonisolated(unsafe) properties during flow isolation.

[hborla]

• Explanation: Flow isolation was not allowing access to nonisolated(unsafe) properties inside actor inits/deinit, leading to warnings in the following code from Cannot access nonisolated(unsafe) var property of actor from init #73294:

  actor A {
    private nonisolated(unsafe) var child: B!
    init() {
      child = B(parent: self) // warning: cannot access property 'child' here in non-isolated initializer; this is an error in the Swift 6 language mode
    }
  }
  actor B {
    unowned nonisolated let parent: any Actor
  
    init(parent: any Actor) {
      self.parent = parent
    }
  }

  This change prevents flow isolation from diagnosing nonisolated(unsafe) properties by checking for ActorIsolation::NonisolatedUnsafe in varIsSafeAcrossActors.

• Scope: Only impacts access to nonisolated(unsafe) properties inside actor inits and deinits.

• Risk: Low; this change allows more code to compile under the Swift 6 language mode, and the only impact in other language modes is fewer warnings under -strict-concurrency=complete. This change has no impact on code that uses -strict-concurrency=minimal.

• Testing: Added a new test.

• Main branch PR: [Concurrency] Don't diagnose nonisolated(unsafe) properties during flow isolation. #73308

[hborla]

@swift-ci please test