IN clauses that are connected by OR require only a single table join. To avoid
extraneous joins, keep track of generated table aliases, and let sibling
clauses piggy-back on those aliases when possible.
Introduces WP_Tax_Query::sanitize_relation() to reduce some repeated code.
Adds unit tests to verify the JOIN consolidation, and integration tests for
cases where JOINS are being combined.
Props boonebgorges, otto42, jakub.tyrcha.
Fixes#18105.
git-svn-id: https://develop.svn.wordpress.org/trunk@29902 602fd350-edb4-49c9-b593-d223f7449a82
56c82e534f