Improve lastBuildDate timestamp in rss feeds

RSS feed timestamps should reflect the actual timestamps for those RSS feeds rather than the generic timestamp for all posts and all comments. 

Props stevenkword.
Fixes #4575.




git-svn-id: https://develop.svn.wordpress.org/trunk@32765 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/feed-rss2-comments.php

@@ -43,7 +43,7 @@ do_action( 'rss_tag_pre', 'rss2-comments' );
 	<atom:link href="<?php self_link(); ?>" rel="self" type="application/rss+xml" />
 	<link><?php (is_single()) ? the_permalink_rss() : bloginfo_rss("url") ?></link>
 	<description><?php bloginfo_rss("description") ?></description>
-	<lastBuildDate><?php echo mysql2date('r', get_lastcommentmodified('GMT')); ?></lastBuildDate>
+	<lastBuildDate><?php echo mysql2date('r', get_last_build_date_feed(), false ); ?></lastBuildDate>
 	<sy:updatePeriod><?php
 		/** This filter is documented in wp-includes/feed-rss2.php */
 		echo apply_filters( 'rss_update_period', 'hourly' );