[fix](planner) insert default value should not change return type of function object in function set (#17536)
function now's return type changed to datetimev2 by mistake.
It can be reproduced in the following way
CREATE TABLE `testdt` (
`c1` int(11) NULL,
`c2` datetimev2 NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=OLAP
DUPLICATE KEY(`c1`, `c2`)
COMMENT 'OLAP'
DISTRIBUTED BY HASH(`c1`) BUCKETS 10
PROPERTIES (
"replication_allocation" = "tag.location.default: 1",
"in_memory" = "false",
"storage_format" = "V2",
"light_schema_change" = "true",
"disable_auto_compaction" = "false"
);
insert into testdt2(c1) values(1);
select now();
678f34cad3