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
