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.


git-svn-id: https://develop.svn.wordpress.org/trunk@33891 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/template.php

@@ -743,8 +743,8 @@ function meta_form( $post = null ) {
  *
  * @since 0.71
  *
- * @global WP_Locale $wp_locale
- * @global object    $comment
+ * @global WP_Locale  $wp_locale
+ * @global WP_Comment $comment
  *
  * @param int|bool $edit      Accepts 1|true for editing the date, 0|false for adding the date.
  * @param int|bool $for_post  Accepts 1|true for applying the date to a post, 0|false for a comment.