title: Implement interface on class as part of a Code Fix
body: <p>As part of a Roslyn code fix, I need to implement an interface on a class, if the class does not already implement it.</p>
  
  <p>So far I'm able to detect whether the class implements the interface:</p>
  
  <pre><code>var implements = false;
  foreach (var b in t.BaseList.Types)
  {
      var name = (b.Type as IdentifierNameSyntax)?.Identifier.ValueText;
      if (name == "IInterfaceName")
      {
          implements = true;
          break;
      }
  }
  </code></pre>
  
  <p>Now, if <code>implements</code> is false, I need to add the interface to the list of basetypes.
  I've tried <code>t.BaseTypes.Add(...)</code> and here I'm a bit stuck - not sure how to construct the right parameter.</p>
  
  <p>Is this the right approach?</p>