SubqueryConstraint doesn't relabel aliases correctly

[Anssi Kääriäinen]

SubqueryConstraint (added in #19385) doesn't correctly handle relabeling of aliases. The problem is that SubqueryConstraint defines relabeled_clone(), but due to SubqueryConstraint being added directly to wherenode as children, not part of a children, the relabeled_clone() isn't ever called.

To trigger this bug one needs to do for example a double __in lookup (that is, lookup with __in=qs where the qs has a __in=qs2 lookup itself), or __in=qs + combine of query.

This can be fixed by adding clone() and relabel_aliases() methods to SubqueryConstraint.

This bug is present in 1.6-alpha-1.

[Anssi Kääriäinen <akaariai@…>]

In d467e117856a7fa6da5e90471144aaa82d822065:

Fixed #20507 -- SubqueryConstraint alias relabeling

The SubqueryConstraint defined relabeled_clone(), but that was never
called. Instead there is now clone() and relabel_aliases() methods for
SubqueryConstraint.

A related problem was that SubqueryConstraint didn't correctly use
quote_name_unless_alias() of the outer query. This resulted in failures
when running under PostgreSQL.

[Anssi Kääriäinen <akaariai@…>]

In 37ea9f9c03b8ec4305ac978e85db653b432b9921:

Fixed #20507 -- SubqueryConstraint alias handling

MySQL should work now, too.