expression,types: improve str_to_date builtin function compatibility

[tiancaiamao]

What problem does this PR solve?

mysql:

mysql> select str_to_date('10/28/2011 9:46:29 Am','%m/%d/%Y %l:%i:%s %p');
+-------------------------------------------------------------+
| str_to_date('10/28/2011 9:46:29 Am','%m/%d/%Y %l:%i:%s %p') |
+-------------------------------------------------------------+
| 2011-10-28 09:46:29                                         |
+-------------------------------------------------------------+
1 row in set (0.00 sec)

tidb:

mysql> select str_to_date('10/28/2011 9:46:29 Am','%m/%d/%Y %l:%i:%s %p');
+-------------------------------------------------------------+
| str_to_date('10/28/2011 9:46:29 Am','%m/%d/%Y %l:%i:%s %p') |
+-------------------------------------------------------------+
| NULL                                                        |
+-------------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

%p formator should accept both AM/PM and am/pm, it's case insensitive

What is changed and how it works?

Tiny fix

Check List

Tests

• Unit test

[tiancaiamao]

PTAL @winkyao @lysu

[codecov]

Codecov Report

Merging #9617 into master will increase coverage by 0.0016%.
The diff coverage is 57.1428%.

@@               Coverage Diff                @@
##             master      #9617        +/-   ##
================================================
+ Coverage   67.3832%   67.3849%   +0.0016%     
================================================
  Files           374        374                
  Lines         78938      78942         +4     
================================================
+ Hits          53191      53195         +4     
  Misses        20994      20994                
  Partials       4753       4753

[shenli]

LGTM
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[lysu]

LGTM

[lysu]

/run-all-tests