VSTHRD010 mis-fires when calling async method

[AArnott]

The VSTHRD010 analyzer will misfire on ShowToolWindow in the sample below when adding the call to the async method that switches itself to the main thread:

        private void ShowToolWindow(object sender, EventArgs e)
        {
            ThreadHelper.JoinableTaskFactory.RunAsync(async delegate
            {
                await FooAsync(); // this line is what adds the VSTHRD010 diagnostic
            });
        }
        
        private async Task FooAsync()
        {
            await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();
        }

This would be a regression from #220 which merged into master and even 9869911 which merged the same into v15.7