Avoid redundant table joins in WP_Tax_Query.

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.
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