Changeset 53749 for trunk/src/wp-includes/wp-db.php

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.