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.
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git-svn-id: http://core.svn.wordpress.org/trunk@29711 1a063a9b-81f0-0310-95a4-ce76da25c4cd