addition between `datetime` and real `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 "1900-01-01 00:00:00" + INTERVAL 1.123456789e4 SECOND;

2. What did you expect to see?

In MySQL:

mysql> SELECT "1900-01-01 00:00:00" + INTERVAL 1.123456789e4 SECOND;
+-------------------------------------------------------+
| "1900-01-01 00:00:00" + INTERVAL 1.123456789e4 SECOND |
+-------------------------------------------------------+
| 1900-01-01 03:07:14.567890                            |
+-------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.15    |
+-----------+
1 row in set (0.00 sec)

3. What did you see instead?

mysql> SELECT "1900-01-01 00:00:00" + INTERVAL 1.123456789e4 SECOND;
+-------------------------------------------------------+
| "1900-01-01 00:00:00" + INTERVAL 1.123456789e4 SECOND |
+-------------------------------------------------------+
| 1900-01-01 03:07:15                                   |
+-------------------------------------------------------+
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)?

mysql> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v3.0.0-beta.1-199-g71d15aa5a
Git Commit Hash: 71d15aa5a3a4bcb47a0111025704d45d0b46b844
Git Branch: master
UTC Build Time: 2019-05-05 08:06:52
GoVersion: go version go1.12 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false
1 row in set (0.00 sec)