MDEV-18813 PROCEDURE and anonymous blocks silently ignore FETCH GROUP NEXT ROW

Part#2 (final): rewritting the code to pass the correct enum_sp_aggregate_type
to the sp_head constructor, so sp_head never changes its aggregation type
later on. The grammar has been simplified and defragmented.
This allowed to check aggregate specific instructions right after
a routine body has been scanned, by calling new LEX methods:
  sp_body_finalize_{procedure|function|trigger|event}()

Moving some C++ code from *.yy to a few new helper methods in LEX.

mysql-test/main/custom_aggregate_functions.test

@@ -965,3 +965,54 @@ select i, sum(i) from t1 group by i with rollup;
 # Cleanup
 drop function agg_sum;
 drop table t1;
+
+
+--echo #
+--echo # MDEV-18813 PROCEDURE and anonymous blocks silently ignore FETCH GROUP NEXT ROW
+--echo #
+
+
+DELIMITER $$;
+--error ER_NOT_AGGREGATE_FUNCTION
+CREATE PROCEDURE p1()
+BEGIN
+  FETCH GROUP NEXT ROW;
+END;
+$$
+DELIMITER ;$$
+
+
+DELIMITER $$;
+--error ER_NOT_AGGREGATE_FUNCTION
+BEGIN NOT ATOMIC
+  FETCH GROUP NEXT ROW;
+END;
+$$
+DELIMITER ;$$
+
+
+DELIMITER $$;
+--error ER_NOT_AGGREGATE_FUNCTION
+CREATE DEFINER=root@localhost  FUNCTION f1() RETURNS INT
+BEGIN
+  FETCH GROUP NEXT ROW;
+  RETURN 0;
+END;
+$$
+DELIMITER ;$$
+
+
+CREATE TABLE t1 (a INT);
+--error ER_NOT_AGGREGATE_FUNCTION
+CREATE TRIGGER tr1
+  AFTER INSERT ON t1 FOR EACH ROW 
+   FETCH GROUP NEXT ROW;
+DROP TABLE t1;
+
+
+--error ER_NOT_AGGREGATE_FUNCTION
+CREATE EVENT ev1
+  ON SCHEDULE EVERY 1 HOUR
+  STARTS CURRENT_TIMESTAMP + INTERVAL 1 MONTH
+  ENDS CURRENT_TIMESTAMP + INTERVAL 1 MONTH + INTERVAL 1 WEEK
+DO FETCH GROUP NEXT ROW;