Coding Standards: Move WP_Object_Cache to wp-includes/class-wp-object-cache.php.

This ensures the file containing the `WP_Object_Cache` class conforms to the coding standards.

For backward compatibility, the new file is included from `wp-includes/cache.php`.

Fixes #49373. See #49222.

git-svn-id: https://develop.svn.wordpress.org/trunk@47197 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov