Can we break cancellation propagation in when(first:) after promise resolution when cancelRemaining is false?

[lilyball]

When using when(first:) without cancelRemaining, the returned Promise is considered a child of all of the input Promises. This means it will prevent cancellation propagation from any other child promises of those inputs. We should see if it's reasonable to "detach" the returned Promise from the input promises after the returned Promise has resolved so e.g. it won't prevent cancellation of any outstanding input promises whose other children have cancelled.

We can't simply propagate cancellation at this point because we don't want to cancel the other inputs if we're the only child.

[lilyball]

Here's a test case that describes the behavior I'm looking for:

    func testWhenAllowCancelPropagationForInputsWithoutCancelRemaining() {
        let sema1 = DispatchSemaphore(value: 1)
        let sema2 = DispatchSemaphore(value: 1)
        sema1.wait()
        sema2.wait()
        // Set up 2 input promises, hang another child off of one, have the 2nd resolve first, then
        // cancel the new child and see if the first propagates cancellation.
        let newChild: Promise<Int,String>
        let promise: Promise<Int,String>
        let promise1CancelExpectation: XCTestExpectation
        do {
            let promise1 = Promise<Int,String>(on: .utility, { (resolver) in
                resolver.onRequestCancel(on: .immediate, { (resolver) in
                    resolver.cancel()
                })
                sema1.wait()
                sema1.signal()
                resolver.fulfill(with: 0xDEADBEEF)
            })
            promise1CancelExpectation = XCTestExpectation(onCancel: promise1)
            let promise2 = Promise<Int,String>(on: .utility, { (resolver) in
                resolver.onRequestCancel(on: .immediate, { (resolver) in
                    resolver.cancel()
                })
                sema2.wait()
                sema2.signal()
                resolver.fulfill(with: 42)
            })
            promise = when(first: [promise1, promise2], cancelRemaining: false)
            newChild = promise1.then(on: .utility, { $0 })
        }
        let expectation = XCTestExpectation(onSuccess: promise, expectedValue: 42)
        sema2.signal()
        wait(for: [expectation], timeout: 1)
        XCTAssertNil(newChild.result)
        newChild.requestCancel()
        wait(for: [promise1CancelExpectation], timeout: 1)
    }

[lilyball]

This looks very non-trivial to do.