parser: fix IF NOT EXISTS support for MariaDB ADD PARTITION syntax

[spongedu]

What problem does this PR solve?

#337 support IF NOT EXISTS option forADD PARTITION syntax in MaraiDB 10.0.2+, but not complete. the same support for ADD PARTITION PARTITIONS NUM syntax is missing.
For example, we have a table defined as follows:

 CREATE TABLE `t2` (
  `name` varchar(30) DEFAULT NULL,
  `started` date DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
PARTITION BY HASH( year(`started`) )
PARTITIONS 6 

in MaraiDB 10.0.2+, the following statement is ok :

MariaDB [test]> alter table t2 add partition if not exists partitions 2;
Query OK, 0 rows affected (0.11 sec)

while for now, this sql failed in parser:

tidb> alter table t2 add partition if not exists partitions 2;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use li
ne 1 column 53 near "partitions 2" 

What is changed and how it works?

Add IfNotExists option for the corresponding branch for ADD PARTITION syntax in parser.y.

Check List

Tests

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

Code changes

N/A

Side effects

N/A

[codecov]

Codecov Report

Merging #455 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #455   +/-   ##
=====================================
  Coverage      71%    71%           
=====================================
  Files          32     32           
  Lines        7522   7522           
=====================================
  Hits         5341   5341           
  Misses       1665   1665           
  Partials      516    516

Impacted Files	Coverage Δ
parser.go	70.58% <ø> (ø)	⬆️

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[spongedu]

@csuzhangxc PTAL

[csuzhangxc]

LGTM

[kennytm]

LGTM