process_comment: Use last defined callback

Instead of using the last registered callback,
iterate through all callbacks and use the last
one that was actually implemented and returned `Some()`.

This is essentially when users register more than one parse callback -
the process_comment may not be the last.

Author: jschwe

bindgen/lib.rs

@@ -582,9 +582,7 @@ impl BindgenOptions {
 
     fn process_comment(&self, comment: &str) -> String {
         let comment = comment::preprocess(comment);
-        self.parse_callbacks
-            .last()
-            .and_then(|cb| cb.process_comment(&comment))
+        self.last_callback(|cb| cb.process_comment(&comment))
             .unwrap_or(comment)
     }
 }