Use only LEFT JOINs when a meta_query contains a NOT EXISTS clause.

Mixing LEFT and INNER JOIN in these cases results in posts with no metadata
being improperly excluded from results.

Props johnrom.
Fixes #29062.
Built from https://develop.svn.wordpress.org/trunk@29890


git-svn-id: http://core.svn.wordpress.org/trunk@29646 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Introduce support for nested queries in WP_Tax_Query.

Previously, tax query arguments could be joined by a single AND or OR relation.
Now, these queries can be arbitrarily nested, allowing clauses to be linked
together with multiple relations.

In a few places, WP_Query runs through a list of clauses in a tax_query in order
to set certain query vars for backward compatibility. The necessary changes have
been made to WP_Query to support this feature with the new complex structure of
tax_query. Unit tests are included for these backward compatibility fixes.

Unit tests for the new nesting syntax are included.

Props boonebgorges.
Fixes #29718. See #29738.
Built from https://develop.svn.wordpress.org/trunk@29891


git-svn-id: http://core.svn.wordpress.org/trunk@29647 1a063a9b-81f0-0310-95a4-ce76da25c4cd