[Concurrency] Don't allow nonisolated initializers to introduce data races via superclass property observers.

[hborla]

Fixes an issue where a nonisolated initializer of a subclass could invoke global-actor-isolated properties in the superclass:

@MainActor
func mainFn() { ... }

@MainActor
class Parent {
  var x: Int = 0 {
    didSet { mainFn() }
  }

  nonisolated init() {}
}

class Child: Parent {
  override nonisolated init() {
    super.init()
    super.x = 10 // this should be an error!
  }
}

The first commit of this change also renames Independent to Nonisolated throughout the actor isolation checker.

Resolves rdar://86550653

[hborla]

@swift-ci please smoke test