Introduce WP_Comment class to model/strongly-type rows from the comments database table. Inclusion of this class is a pre-req for some more general comment cleanup and sanity.

* Takes inspiration from `WP_Post` and adds sanity to comment caching. 
* Clarifies when the current global value for `$comment` is returned. The current implementation in `get_comment()` introduces side effects and an occasion stale global value for `$comment` when comment caches are cleaned.
* Strongly-types `@param` docs
* This class is marked `final` for now

Props wonderboymusic, nacin.

See #32619.

Built from https://develop.svn.wordpress.org/trunk@33891


git-svn-id: http://core.svn.wordpress.org/trunk@33860 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/edit-form-comment.php

@@ -171,7 +171,7 @@ do_action( 'add_meta_boxes', 'comment', $comment );
  *
  * @since 3.0.0
  *
- * @param object $comment Comment object.
+ * @param WP_Comment $comment Comment object.
  */
 do_action( 'add_meta_boxes_comment', $comment );