Add a matcher for decls that has a specific attribute.
If we have this matcher, do the specialized ones still provide enough value? I was expecting us to either go for the specialized ones, or provide a generic one. Having both in the interface seems odd...
And now that I see how this affects the dynamic matchers, I wonder how we're going to fix that up - that is, we'll need to teach the dynamic matchers to understand the possible clang attributes somehow.