Produce warning when async work isn't tracked

[AArnott]

If ThreadHelper.JoinableTaskFactory.RunAsync is called and the JoinableTask returned isn't tracked anywhere, that represents unmonitored async work that can crash if still running when VS shuts down the CLR. Folks should follow a pattern of tracking their async work.
This is actually more general to all async methods invoked but not awaited or assigned to something.

Note that some JTF instances are associated with a JoinableTaskCollection that tracks the async work implicitly and that shouldn't produce a warning.

Maybe this should go into the vs-threading library, except perhaps not all apps are as sensitive to untracked async work as VS is. Also, we may want to special case well-known JTF instances such as the one on ThreadHelper to know that that is not a tracking one and should therefore be flagged.