Problem with usage of WP_Term_Query inside WP_Tax_Query

[strategio]

This problem started on WP 4.9 because of #37038.

In WPML plugin, we filter the tax queries in order to convert the term IDs to their translations in the current language (if any).

In WP 4.8.3, WP_Tax_Query::transform_query was making direct calls to the DB.

In WP 4.9, WP_Tax_Query::transform_query is using WP_Term_Query to fetch the terms of the meta query.

The problem is that we have filters on WP_Term_Query (on get_terms_args) which applies to this.

This is usually not a problem, except for a post query with tax queries and suppress_filters set to true.

Here's an example:

$args = array(
    'post_type' => 'post',
    'suppress_filters' => true,
    'tax_query' => array(
        array(
            'taxonomy' => 'category',
            'field'    => 'term_id',
            'terms'    => [ 3, 4 ] // 3 is the English category and 4 is the translation in French
        ),
    )
);

$wp_query = new WP_Query( $args );

What is expected (and actual behavior in 4.8.3): This query should return all the posts attached to the category 3 or 4, whatever the current language.

What we have now: The query returns the posts attached to 3 OR the posts attached to 4 (depending on the current language).

So we need a way to flag that WP_Term_Query is used inside WP_Query (via WP_Tax_Query) and suppress_filters is set to true.