ddl: fix incorrect origin default bit value in ColumnInfo (#12168)

[sre-bot]

cherry-pick #12168 to release-3.0

---

What problem does this PR solve?

create table t (a int);
insert into t set a=1;
alter table `t` add column c bit(16) NULL default b'1100110111001';
update t set c = b'11100000000111';
(1690, u"BIT value is out of range in '(16)'")

select tidb_version();
+-------------------------------------------------------------------+
| tidb_version()                                                    |
+-------------------------------------------------------------------+
| Release Version: v4.0.0-alpha-163-g5352e7c65                      |
| Git Commit Hash: 5352e7c65c3c200bef65d6897840ffa322b0650e         |
| Git Branch: master                                                |
| UTC Build Time: 2019-09-02 02:35:45                               |
| GoVersion: go version go1.12.9 darwin/amd64                       |
| Race Enabled: false                                               |
| TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306 |
| Check Table Before Drop: false                                    |
+-------------------------------------------------------------------+

What is changed and how it works?

Add a conversion in the getter function of ColumnInfo.OriginDefaultValue.

Check List

Tests

• Unit test
• Integration test

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix incorrect origin default bit value in ColumnInfo.

[sre-bot]

/run-all-tests

[lonng]

LGTM

[zimulala]

LGTM

[sre-bot]

/run-all-tests

[sre-bot]

@sre-bot merge failed.