Implement OR REPLACE option for CREATE AGGREGATE.

Aggregates have acquired a dozen or so optional attributes in recent
years for things like parallel query and moving-aggregate mode; the
lack of an OR REPLACE option to add or change these for an existing
agg makes extension upgrades gratuitously hard. Rectify.

src/backend/commands/aggregatecmds.c

@@ -54,7 +54,12 @@ static char extractModify(DefElem *defel);
  * "parameters" is a list of DefElem representing the agg's definition clauses.
  */
 ObjectAddress
-DefineAggregate(ParseState *pstate, List *name, List *args, bool oldstyle, List *parameters)
+DefineAggregate(ParseState *pstate,
+				List *name,
+				List *args,
+				bool oldstyle,
+				List *parameters,
+				bool replace)
 {
 	char	   *aggName;
 	Oid			aggNamespace;
@@ -436,6 +441,7 @@ DefineAggregate(ParseState *pstate, List *name, List *args, bool oldstyle, List
 	 */
 	return AggregateCreate(aggName, /* aggregate name */
 						   aggNamespace,	/* namespace */
+						   replace,
 						   aggKind,
 						   numArgs,
 						   numDirectArgs,