Fix bug in HashAgg's selective-column-spilling logic.

Commit 23023022 taught nodeAgg.c that, when spilling tuples from
memory in an oversized hash aggregation, it only needed to spill
input columns referenced in the node's tlist and quals.  Unfortunately,
that's wrong: we also have to save the grouping columns.  The error
is masked in common cases because the grouping columns also appear
in the tlist, but that's not necessarily true.  The main category
of plans where it's not true seem to come from semijoins ("WHERE
outercol IN (SELECT innercol FROM innertable)") where the innercol
needs an implicit promotion to make it comparable to the outercol.
The grouping column will be "innercol::promotedtype", but that
expression appears nowhere in the Agg node's own tlist and quals;
only the bare "innercol" is found in the tlist.

I spent quite a bit of time looking for a suitable regression test
case for this, without much success.  If the number of distinct
values of the innercol is large enough to make spilling happen,
the planner tends to prefer a non-HashAgg plan, at least for
problem sizes that are reasonable to use in the regression tests.
So, no new regression test.  However, this patch does demonstrably
fix the originally-reported test case.

Per report from s.p.e (at) gmx-topmail.de.  Backpatch to v13
where the troublesome code came in.

Discussion: https://postgr.es/m/trinity-1c565d44-159f-488b-a518-caf13883134f-1611835701633@3c-app-gmx-bap78

src/backend/executor/nodeAgg.c

@@ -1391,7 +1391,7 @@ project_aggregates(AggState *aggstate)
 }
 
 /*
- * Walk tlist and qual to find referenced colnos, dividing them into
+ * Find input-tuple columns that are needed, dividing them into
  * aggregated and unaggregated sets.
  */
 static void
@@ -1404,9 +1404,15 @@ find_cols(AggState *aggstate, Bitmapset **aggregated, Bitmapset **unaggregated)
 	context.aggregated = NULL;
 	context.unaggregated = NULL;
 
+	/* Examine tlist and quals */
 	(void) find_cols_walker((Node *) agg->plan.targetlist, &context);
 	(void) find_cols_walker((Node *) agg->plan.qual, &context);
 
+	/* In some cases, grouping columns will not appear in the tlist */
+	for (int i = 0; i < agg->numCols; i++)
+		context.unaggregated = bms_add_member(context.unaggregated,
+											  agg->grpColIdx[i]);
+
 	*aggregated = context.aggregated;
 	*unaggregated = context.unaggregated;
 }