when(first:cancelRemaining:) won't cancel synchronously if all inputs have cancelled

[lilyball]

From reading the code it appears that when(first: [Promise(with: .cancelled)]) returns a pending promise that will be cancelled asynchronously (on the .utility queue). We should fix this so it synchronously cancels if all inputs were already cancelled. This will make it consistent because it already fulfills/rejects synchronously if any input has been fulfilled/rejected.