Print diagnostic groups as part of the LLVM printer in the same manner as the
Swift one does, always. Make `-print-diagnostic-groups` an inert option, since we
always print diagnostic group names with the `[#GroupName]` syntax.
As part of this, we no longer render the diagnostic group name as part
of the diagnostic *text*, instead leaving it up to the diagnostic
renderer to handle the category appropriately. Update all of the tests
that were depending on `-print-diagnostic-groups` putting it into the
text to instead use the `{{documentation-file=<file name>}}`
diagnostic verification syntax.
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
