title: How to modify Solution/Project object?
body: <p>I have defined attribute MyAttribute : Attribute that is supposed to be used exactly once within a class (only one constructor per class may have it and it can appear only once on ctor’s attribute list).</p>
  
  <p>I have created a Roslyn analyzer to check this which marks every usage of such attribute (if used more than once) and allows user to pick fixture called "Leave this attribute occurrence and delete all others".</p>
  
  <p>Now within FixProvider I need to return new modified Solution. It's not difficult to modify every Document that requires the fix (by using SyntaxRewriter to modify SyntaxTree inside). However, I have no idea how to modify Solution or Project - they don't have any method like "ReplaceProject"/"ReplaceDocument".</p>
  
  <p>How to do that?</p>