Check out this blog post, the macro provides two functionalities - one is collapse all XML comments, the other one is toggle all XML comments.
The blog post mentioned that the macro should work for Visual Studio 2005 and above so I suppose it will work in VS 2012 too. I am using VS 2010 and it is working great for me even after I hook the macro up with keyboard shortcuts. I am glad that I found the blog post link in one of the answer in another stack overflow question =)