Under the task-to-thread model, specifying a priority doesn't make
sense.
Here, variations of addTask and addTaskUnlessCancelled are introduced
which do not take a priority. Additionally, the original functions are
made unavailable.
In the task-to-thread model, there are no threading mechanisms by which
work could be offloaded onto another thread. As such, callback based
asynchronous APIs which are not Swift async do not make sense.
rdar://99047747