Fix orderby meta handling for WP_Term_Query.

In order to allow meta-related values of `orderby` to be handled properly,
the term query's `meta_query` object must run its `get_sql()` method before
`orderby` parsing.

Fixing this bug required addressing another bug in `WP_Meta_Query`, which
caused the table alias index not to be reset when calling `get_sql()`
multiple times on the same object.

Props littler.chicken.
Fixes #37151.

git-svn-id: https://develop.svn.wordpress.org/trunk@37860 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-meta-query.php

@@ -327,6 +327,8 @@ class WP_Meta_Query {
 			return false;
 		}
 
+		$this->table_aliases = array();
+
 		$this->meta_table     = $meta_table;
 		$this->meta_id_column = sanitize_key( $type . '_id' );