Coding Standards: Move wp-includes/wp-db.php to wp-includes/class-wpdb.php.

This renames the file containing the `wpdb` class to conform to the coding standards.

This commit also includes:
* A new `wp-db.php` that loads the new file, for anyone that may have been including the file directly.
* Replacing references to the old filename with the new filename.

Fixes #56268. See #55647.
Built from https://develop.svn.wordpress.org/trunk@53749


git-svn-id: http://core.svn.wordpress.org/trunk@53308 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/install.php

@@ -42,7 +42,7 @@ require_once ABSPATH . 'wp-admin/includes/upgrade.php';
 require_once ABSPATH . 'wp-admin/includes/translation-install.php';
 
 /** Load wpdb */
-require_once ABSPATH . WPINC . '/wp-db.php';
+require_once ABSPATH . WPINC . '/class-wpdb.php';
 
 nocache_headers();