Fix handling of argument and result datatypes for partial aggregation.

When doing partial aggregation, the args list of the upper (combining)
Aggref node is replaced by a Var representing the output of the partial
aggregation steps, which has either the aggregate's transition data type
or a serialized representation of that.  However, nodeAgg.c blindly
continued to use the args list as an indication of the user-level argument
types.  This broke resolution of polymorphic transition datatypes at
executor startup (though it accidentally failed to fail for the ANYARRAY
case, which is likely the only one anyone had tested).  Moreover, the
constructed FuncExpr passed to the finalfunc contained completely wrong
information, which would have led to bogus answers or crashes for any case
where the finalfunc examined that information (which is only likely to be
with polymorphic aggregates using a non-polymorphic transition type).

As an independent bug, apply_partialaggref_adjustment neglected to resolve
a polymorphic transition datatype before assigning it as the output type
of the lower-level Aggref node.  This again accidentally failed to fail
for ANYARRAY but would be unlikely to work in other cases.

To fix the first problem, record the user-level argument types in a
separate OID-list field of Aggref, and look to that rather than the args
list when asking what the argument types were.  (It turns out to be
convenient to include any "direct" arguments in this list too, although
those are not currently subject to being overwritten.)

Rather than adding yet another resolve_aggregate_transtype() call to fix
the second problem, add an aggtranstype field to Aggref, and store the
resolved transition type OID there when the planner first computes it.
(By doing this in the planner and not the parser, we can allow the
aggregate's transition type to change from time to time, although no DDL
support yet exists for that.)  This saves nothing of consequence for
simple non-polymorphic aggregates, but for polymorphic transition types
we save a catalog lookup during executor startup as well as several
planner lookups that are new in 9.6 due to parallel query planning.

In passing, fix an error that was introduced into count_agg_clauses_walker
some time ago: it was applying exprTypmod() to something that wasn't an
expression node at all, but a TargetEntry.  exprTypmod silently returned
-1 so that there was not an obvious failure, but this broke the intended
sensitivity of aggregate space consumption estimates to the typmod of
varchar and similar data types.  This part needs to be back-patched.

Catversion bump due to change of stored Aggref nodes.

Discussion: <8229.1466109074@sss.pgh.pa.us>

src/include/nodes/primnodes.h

@@ -256,6 +256,18 @@ typedef struct Param
  * The direct arguments appear in aggdirectargs (as a list of plain
  * expressions, not TargetEntry nodes).
  *
+ * aggtranstype is the data type of the state transition values for this
+ * aggregate (resolved to an actual type, if agg's transtype is polymorphic).
+ * This is determined during planning and is InvalidOid before that.
+ *
+ * aggargtypes is an OID list of the data types of the direct and regular
+ * arguments.  Normally it's redundant with the aggdirectargs and args lists,
+ * but in a combining aggregate, it's not because the args list has been
+ * replaced with a single argument representing the partial-aggregate
+ * transition values.
+ *
+ * XXX need more documentation about partial aggregation here
+ *
  * 'aggtype' and 'aggoutputtype' are the same except when we're performing
  * partal aggregation; in that case, we output transition states.  Nothing
  * interesting happens in the Aggref itself, but we must set the output data
@@ -272,6 +284,8 @@ typedef struct Aggref
 	Oid			aggoutputtype;	/* type Oid of result of this aggregate */
 	Oid			aggcollid;		/* OID of collation of result */
 	Oid			inputcollid;	/* OID of collation that function should use */
+	Oid			aggtranstype;	/* type Oid of aggregate's transition value */
+	List	   *aggargtypes;	/* type Oids of direct and aggregated args */
 	List	   *aggdirectargs;	/* direct arguments, if an ordered-set agg */
 	List	   *args;			/* aggregated arguments and sort expressions */
 	List	   *aggorder;		/* ORDER BY (list of SortGroupClause) */