AJAX: add a new function, wp_doing_ajax(), which can replace... (wait for it...) DOING_AJAX checks via the constant.

Props Mte90, sebastian.pisula, swissspidy.
Fixes #25669.

Built from https://develop.svn.wordpress.org/trunk@38334


git-svn-id: http://core.svn.wordpress.org/trunk@38275 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-ajax-response.php

@@ -149,7 +149,7 @@ class WP_Ajax_Response {
 		foreach ( (array) $this->responses as $response )
 			echo $response;
 		echo '</wp_ajax>';
-		if ( defined( 'DOING_AJAX' ) && DOING_AJAX )
+		if ( wp_doing_ajax() )
 			wp_die();
 		else
 			die();