UnnecessaryAsync analyzer becomes smarter with dataflow analysis

[semihokur]

Implemented a depth-limited dataflow analysis to get the list of the variable names accessed in the await statement. This is needed to understand whether there are any variables created via using statement are accessed in the await statement. If there is one, we cannot remove async&await keywords as it can cause ObjectDisposedException.

In the example below, AsyncFixer can see that the last await statement involves the disposable object, stream created via using statement.

async Task foo()
{
    using var stream = new MemoryStream();
    int streamOperation()
    {
        return stream.Read(null);
    }

    Task t = Task.Run(() => streamOperation());
    await t;
}

However, in the example below, it is perfectly safe to remove async&await keywords:

async Task foo()
{
    using var stream = new MemoryStream();
    stream.Read(null);
    await Task.Delay(1000);
}