Fix asymmetry in setting EquivalenceClass.ec_sortref

Fix asymmetry in setting EquivalenceClass.ec_sortref

0452b461bc made get_eclass_for_sort_expr() always set
EquivalenceClass.ec_sortref if it's not done yet.  This leads to an asymmetric
situation when whoever first looks for the EquivalenceClass sets the
ec_sortref.  It is also counterintuitive that get_eclass_for_sort_expr()
performs modification of data structures.

This commit makes make_pathkeys_for_sortclauses_extended() responsible for
setting EquivalenceClass.ec_sortref.  Now we set the
EquivalenceClass.ec_sortref's needed to explore alternative GROUP BY ordering
specifically during building pathkeys by the list of grouping clauses.

Discussion: https://postgr.es/m/17037754-f187-4138-8285-0e2bfebd0dea%40postgrespro.ru
Reported-by: Tom Lane
Author: Andrei Lepikhov
Reviewed-by: Alexander Korotkov, Pavel Borisov