Fix failure to mark all aggregates with appropriate transtype.

In commit 915b703e1 I gave get_agg_clause_costs() the responsibility of
marking Aggref nodes with the appropriate aggtranstype.  I failed to notice
that where it was being called from, it might see only a subset of the
Aggref nodes that were in the original targetlist.  Specifically, if there
are duplicate aggregate calls in the tlist, either make_sort_input_target
or make_window_input_target might put just a single instance into the
grouping_target, and then only that instance would get marked.  Fix by
moving the call back into grouping_planner(), before we start building
assorted PathTargets from the query tlist.  Per report from Stefan Huehner.

Report: <20160702131056.GD3165@huehner.biz>

src/test/regress/expected/limit.out

@@ -296,3 +296,27 @@ order by s2 desc;
   0 |  0
 (3 rows)
 
+-- test for failure to set all aggregates' aggtranstype
+explain (verbose, costs off)
+select sum(tenthous) as s1, sum(tenthous) + random()*0 as s2
+  from tenk1 group by thousand order by thousand limit 3;
+                                                    QUERY PLAN                                                     
+-------------------------------------------------------------------------------------------------------------------
+ Limit
+   Output: (sum(tenthous)), (((sum(tenthous))::double precision + (random() * '0'::double precision))), thousand
+   ->  GroupAggregate
+         Output: sum(tenthous), ((sum(tenthous))::double precision + (random() * '0'::double precision)), thousand
+         Group Key: tenk1.thousand
+         ->  Index Only Scan using tenk1_thous_tenthous on public.tenk1
+               Output: thousand, tenthous
+(7 rows)
+
+select sum(tenthous) as s1, sum(tenthous) + random()*0 as s2
+  from tenk1 group by thousand order by thousand limit 3;
+  s1   |  s2   
+-------+-------
+ 45000 | 45000
+ 45010 | 45010
+ 45020 | 45020
+(3 rows)
+

src/test/regress/sql/limit.sql

@@ -91,3 +91,11 @@ order by s2 desc;
 
 select generate_series(0,2) as s1, generate_series((random()*.1)::int,2) as s2
 order by s2 desc;
+
+-- test for failure to set all aggregates' aggtranstype
+explain (verbose, costs off)
+select sum(tenthous) as s1, sum(tenthous) + random()*0 as s2
+  from tenk1 group by thousand order by thousand limit 3;
+
+select sum(tenthous) as s1, sum(tenthous) + random()*0 as s2
+  from tenk1 group by thousand order by thousand limit 3;