[Concurrency] Downgrade actor_isolated_mutating_func to a warning in a narrow case that was previously accepted.

[hborla]

The following code is accepted by the Swift 5.9 compiler and below:

extension Optional {
    mutating func mutate() async {}
}

actor A {
  var dict: [Int: Int] = [:]

  func test() async {
    await dict[0]. mutate()
  }
}

This is invalid because an actor-isolated value is passed as inout to an async function. However, because this code was previously accepted as valid, this change downgrades the diagnostic to a warning prior to Swift 6.

Resolves rdar://117357457

[hborla]

@swift-ci please smoke test