select_statement: Fix GROUP BY on indexed select
select_statement: Fix GROUP BY on indexed select

Before the change, GROUP BY SELECTs with some WHERE restrictions on an 
indexed column would return invalid results (same grouped column values
appearing multiple times):

CREATE TABLE ks.t(pk int, ck int, v int, PRIMARY KEY(pk, ck));
CREATE INDEX ks_t on ks.t(v);
INSERT INTO ks.t(pk, ck, v) VALUES (1, 2, 3);
INSERT INTO ks.t(pk, ck, v) VALUES (1, 4, 3);
SELECT pk FROM ks.t WHERE v=3 GROUP BY pk;
 pk
----
  1
  1

This is fixed by correctly passing _group_by_cell_indices to 
result_set_builder. Fixes the third failing example from issue #7355.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla