Make orderby=meta_value compatible with meta_query when relation=OR.

Passing orderby=meta_value and meta_key=foo to WP_Query should require that
each matched post have a meta value with the key 'foo'. To make this
requirement compatible with meta_query params that have the relation OR, we
nest the meta_query param, and join it using AND to a meta_query clause
generated from the meta_key/meta_compare/meta_type query vars.

Fixes #25538.

git-svn-id: https://develop.svn.wordpress.org/trunk@29964 602fd350-edb4-49c9-b593-d223f7449a82