Fix SQL generation when `meta_query` has an `'relation' => 'OR'` for its queries and wants to `'orderby' => 'meta_value'`.

Adds unit test.

Props jackreichert.
Fixes #25538.



git-svn-id: https://develop.svn.wordpress.org/trunk@28659 602fd350-edb4-49c9-b593-d223f7449a82