VSTHRD002 should not fire in continuations

[sharwell]

The following code reports VSTHRD002, but it should not:

Task<int> task = default;
task.ContinueWith(t => t.Result); // t is known to be completed at the point of t.Result

[AArnott]

There are other cases, like:

await task;
if (task.Result) ...

or

if (task.IsCompleted) {
   var r = task.Result;
}

In general, until Roslyn has flow analysis APIs available to analyzers I don't think we can cover them all. But ContinueWith looks like it's something we can check for pretty easily.