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.

git-svn-id: https://develop.svn.wordpress.org/trunk@29891 602fd350-edb4-49c9-b593-d223f7449a82