C#: Add default taint step from an implicit operator call to its argument.

[michaelnebel]

Consider this flow source row:

extensions:
  - addsTo:
      pack: codeql/csharp-all
      extensible: sourceModel
    data:
      - ["System.Net.WebSockets", "WebSocket", True, "ReceiveAsync", "", "", "Argument[0]", "remote", "manual"]

and this snippet:

var buffer = new byte[1024];
var result = await webSocket.ReceiveAsync(buffer, System.Threading.CancellationToken.None);

On the second line there's a call to operator an implicit conversion which means that buffer is not an argument to webSocket.ReceiveAsync, but rather an argument to the call to an implicit conversion (which is then an argument to ReceiveAsync). However, we would like buffer to be tainted after the call to ReceiveAsync.

This is solved by adding a reverse update default taint step for post-update nodes for implicit conversion calls.
That is, for f(implicit_conv(x)) we add a step from the post-update of implicit_conv(x) to x.

[michaelnebel]

@hvitved : Is this an acceptable approach for solving the problem?