Tax query transformations unintentionally filtered by 'terms_clauses'

[Chouby]

Since [40918] tax queries are transformed using WP_Term_Query. That's nice because the previously uncached query is now cached.

This however raises a backward compatibility issue as all existing filters applied to a term query are now unintentionnally applied to the tax query transformation.

The same kind of issue was raised by [38667], when the usage of WP_Term_Query was introduced in wp_get_object_terms(). However in this latest case, the unique presence of $args['object_ids'] in the arguments, not interpreted by WP_Term_Query, allows to differentiate term queries fired wp_get_object_terms(). There is no such unique argument which could serve the same purpose for the tax_query transformation.

I suggest to add a dummy parameter to the arguments of the query which would not be interpreted by the core and would only be useful to plugins to distinguish this query from others.

[Chouby]

Add a new filter transform_query_args

[Chouby]

To be consistent with what has been done with the filter wp_get_object_terms_args and the filter get_term_by_args I proposed in #54534 and term_exists_args I proposed in https://core.trac.wordpress.org/ticket/36949#comment:35, I propose to handle the issue by introducing a new filter transform_query_args.