addition between time and interval is not compatible with Mysql

[erjiaqing]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

SELECT time('12:34:56') + INTERVAL 1 SECOND;

2. What did you expect to see?
   In Mysql

mysql> select time('12:34:56') + interval 1 second;
+--------------------------------------+
| time('12:34:56') + interval 1 second |
+--------------------------------------+
| 12:34:57                             |
+--------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead?
   In TiDB

mysql> select time('12:34:56') + interval 1 second;
+--------------------------------------+
| time('12:34:56') + interval 1 second |
+--------------------------------------+
| 2019-04-30 12:34:57                  |
+--------------------------------------+
1 row in set (0.00 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   tidb_version(): Release Version: v3.0.0-beta-463-g36bb1ae89

In mysql's document:

time('12:34:56') + interval 1 second is same as date_add(time('12:34:56'), interval 1 second)

And

The date argument specifies the starting date or datetime value. expr is an expression specifying the interval value to be added or subtracted from the starting date.

[erjiaqing]

dup #9813