[Concurrency] Fix a false-positive metatype capture diagnostic.

[hborla]

Don't diagnose metatype captures in closures that are isolated to the same actor as the enclosing context, because the metatype is not used concurrently. For example, the following code is safe even if the conformance to P is isolated because T never crosses an isolation boundary:

protocol P {}

func acceptClosure(_: () -> Void) {}

@MainActor
func f<T: P>(_: T.Type) {
  acceptClosure {
    Task {
      _ = T.self // okay to capture T.Type in this closure.
    }
  }
}

[hborla]

@swift-ci please smoke test

[DougGregor]

Thank you! LGTM

[hborla]

@swift-ci please smoke test