Support 'EXISTS' and 'NOT EXISTS' in `WP_Tax_Query`.

These new values for the 'operator' parameter make it possible to filter items
that have no term from a given taxonomy, or any term from a given taxonomy.

Includes unit tests.

Fixes #29181.
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git-svn-id: http://core.svn.wordpress.org/trunk@29651 1a063a9b-81f0-0310-95a4-ce76da25c4cd