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/include/commands/defrem.h

@@ -94,7 +94,7 @@ extern void UpdateStatisticsForTypeChange(Oid statsOid,
 
 /* commands/aggregatecmds.c */
 extern ObjectAddress DefineAggregate(ParseState *pstate, List *name, List *args, bool oldstyle,
-				List *parameters);
+									 List *parameters, bool replace);
 
 /* commands/opclasscmds.c */
 extern ObjectAddress DefineOpClass(CreateOpClassStmt *stmt);