Fixed #21376 -- New implementation for query join promotion logic

This commit introduced a new class JoinPromoter that can be used to
abstract away join promotion problems for complex filter conditions.
Query._add_q() and Query.combine() now use the new class.

Also, added a lot of comments about why join promotion is done the way
it is.

Thanks to Tim Graham for original report and testing the changes, and
for Loic Bistuer for review.

django/db/models/sql/expressions.py

@@ -60,6 +60,7 @@ def prepare_leaf(self, node, query, allow_joins):
                 field, sources, opts, join_list, path = query.setup_joins(
                     field_list, query.get_meta(),
                     query.get_initial_alias(), self.reuse)
+                self._used_joins = join_list
                 targets, _, join_list = query.trim_joins(sources, join_list, path)
                 if self.reuse is not None:
                     self.reuse.update(join_list)