types: fix bug which Float type is not effective in AddDate & SubDate function

[lzmhhh123]

What problem does this PR solve?

Before this pr, there are some bugs in AddDate & SubDate function like follow

tidb(localhost:4000) > SELECT TIME('20:20:20.1') + INTERVAL .6 SECOND;
ERROR 2013 (HY000): Lost connection to MySQL server during query

tidb(localhost:4000) > select adddate("2011-01-01 00:00:00", interval 10.1 SECOND);
+------------------------------------------------------+
| adddate("2011-01-01 00:00:00", interval 10.1 SECOND) |
+------------------------------------------------------+
| 2011-01-01 00:00:10                                  |
+------------------------------------------------------+
1 row in set (0.00 sec)

What is changed and how it works?

After this pr, the queries are like follow

tidb(localhost:4000) > SELECT TIME('20:20:20.1') + INTERVAL .6 SECOND;
+-----------------------------------------+
| TIME('20:20:20.1') + INTERVAL .6 SECOND |
+-----------------------------------------+
| 2018-10-08 20:20:20.700000              |
+-----------------------------------------+
1 row in set (0.01 sec)

tidb(localhost:4000) > select adddate("2011-01-01 00:00:00", interval 10.1 SECOND);
+------------------------------------------------------+
| adddate("2011-01-01 00:00:00", interval 10.1 SECOND) |
+------------------------------------------------------+
| 2011-01-01 00:00:10.100000                           |
+------------------------------------------------------+
1 row in set (0.00 sec)

All these results are same as MySQL.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Code changes

• Has exported function/method change
• Has exported variable/fields change

[ngaut]

Could you extract a function to check if we should use Decimal math, there are some redundant code, and it's easy to make mistake to write compose equal condition.

[zz-jason]

LGTM

[zz-jason]

@XuHuaiyu @tiancaiamao PTAL

[tiancaiamao]

/run-all-tests

[tiancaiamao]

/run-all-tests

[tiancaiamao]

/run-all-tests

[tiancaiamao]

LGTM