The `JoinableTaskContext` constructor unfortunately has a poor design where folks passing in `null` for the `SynchronizationContext` may mistakenly believe that they're getting a no-op instance when they aren't, if `SynchronizationContext.Current != null`.
In this change, I add a method that *definitely* does what they expect.
I also add an analyzer and a code fix provider to help identify code that was likely written with the incorrect expectation to help them switch to the new method which matches their expectation, or suppress the warning with a more clear syntax.
d162c02687