*: fix unexpected error when setting collate for database (#16283)

[sre-bot]

cherry-pick #16283 to release-4.0

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What problem does this PR solve?

Problem Summary:
This PR tries to fix the two problems related to the charset and collate when creating a schema:

tidb> create database db5 collate ascii_bin;
ERROR 1115 (42000): Unknown character set: ''

and

tidb> create database test1 charset utf8mb4 collate utf8_general_ci;
ERROR 1115 (42000): Unknown character set: 'utf8mb4'

What is changed and how it works?

What's Changed:
Resolve charset and collation in CreateSchema() function in ddl_api.go

Some charset/collation functions are also refactored to make the logic clear enough.

How it Works:

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Integration test

Side effects

• NA

Release note

• Fix some unexpected errors for specifying charset/collate for CREATE SCHEMA syntax, fix missing COLLATE part for SHOW CREATE SCHEMA syntax.

[sre-bot]

/run-all-tests

[bb7133]

/run-all-tests

[bb7133]

PTAL @Deardrops @zimulala @crazycs520

[Deardrops]

LGTM

[bb7133]

Failed test case:

[2020-04-20T11:10:43.301Z] Statement: #1381 -  SELECT `col_int_unsigned_key` NOT BETWEEN ( 'n' NOT IN ( ( ( COALESCE( 'sphsdfahvhbjmqjiguqqdhpryejduyzihebhanfrwvfojpiedbejnzhguahltffhxasommudcjjelrjyweufrpefinevxukyflrjvgdnopqfrwstmvnhmhcvgjfnnkbfxqcuhekgleangqjc', `col_char_255_key`, 5762637198197260288, ( 0 NOT BETWEEN 'f' AND -7138205409382236160 ), ( '2021-10-02 23:33:21.029368' <=> -10324 ) ) ) IS TRUE ), ( `col_binary_8_key` = ( `col_float_key` <= `col_bigint_key` ) ), ( `col_double_unsigned_key` <= 0 ), 'w', `col_bigint` ) ) AND ( `col_double` BETWEEN `col_varbinary_32_key` AND ( ( `col_date_key` <=> ( ISNULL( ( `col_tinyint_unsigned` NOT LIKE 7542403475938738176 ) ) ) ) <> 6315735527433699328 ) ) AS field1 FROM `table1000_int_autoinc` WHERE LEAST( '2017-09-08', ( COALESCE( ( STRCMP( 0, `col_decimal` ) ), ( STRCMP( ( `col_decimal_unsigned` <=> 2833045640592752640 ), `col_int_key` ) ) ) ) ) ORDER BY field1 LIMIT 3 /* QNO 1383 CON_ID 196 */ ;
[2020-04-20T11:10:43.301Z] NoPushDown Output: 
[2020-04-20T11:10:43.301Z] field1
[2020-04-20T11:10:43.301Z] NULL
[2020-04-20T11:10:43.301Z] NULL
[2020-04-20T11:10:43.301Z] NULL
[2020-04-20T11:10:43.301Z] 
[2020-04-20T11:10:43.301Z] 
[2020-04-20T11:10:43.301Z] WithPushDown Output: 
[2020-04-20T11:10:43.301Z] Error 1105: runtime error: index out of range [0] with length 0
[2020-04-20T11:10:43.301Z] 

https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/tidb_ghpr_integration_copr_test/detail/tidb_ghpr_integration_copr_test/3870/pipeline

[zimulala]

LGTM

[sre-bot]

Your auto merge job has been accepted, waiting for:

• 16710

[sre-bot]

/run-all-tests

[sre-bot]

@sre-bot merge failed.

[zz-jason]

LGTM

[zz-jason]

/merge

[sre-bot]

Your auto merge job has been accepted, waiting for:

• 16651
• 16699

[sre-bot]

/run-all-tests

[sre-bot]

@sre-bot merge failed.