Ensure assignment to a discard produces VSTHRD110

[sharwell]

Is your feature request related to a problem? Please describe.

The following code should produce a warning. Currently Roslyn is using discards in a few places to address CS4014 prior to updating the code to enforce VSTHRD110 (a more strict analyzer with the same intent). We should ensure that VSTHRD110 is reported for this scenario.

async Task Method()
{
  _ = Task.Run(() => 3); // VSTHRD110: Observe result of async calls
}

[AArnott]

As that is a common way for folks to suppress the warning when they really don't want it tracked, I'm not sure why we want to defeat it. We offer a .Forget() extension method for this purpose as well.