WIP:types: fix isDateFormat function bugs

[mengxin9014]

What problem does this PR solve?

Issue Number: close #27573

Problem Summary:
fix the bug that a string like "20129" will not be judged as date type
change date_add and date_sub string_(int/string/real/decimal) function return type to string

What is changed and how it works?

Proposal: xxx

What's Changed:
time.go
expression/builtin_time.go
expression/builtin_time_test.go
expression/builtin_time_vec_generated.go
types/time_test.go
cmd/explaintest/r/tpch.result

How it Works:
if the length of a string with no separator is 5, it will be date type
if use date_add or date_sub string_(int/string/real/decimal) function will return string type

Check List

Tests

• Unit test

Release note

change date_add and date_sub string_(int/string/real/decimal) function return type to string.

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[mengxin9014]

/run-all-tests

[wjhuang2016]

The returning type is wrong.
TiDB:

mysql> select DATE_ADD(a, INTERVAL 1 day ) from t ;
Field   1:  `DATE_ADD(a, INTERVAL 1 day )`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       DATETIME
Collation:  binary (63)
Length:     29
Max_length: 19
Decimals:   31
Flags:      NOT_NULL BINARY


+------------------------------+
| DATE_ADD(a, INTERVAL 1 day ) |
+------------------------------+
| 2020-12-10 00:00:00          |
+------------------------------+
1 row in set (0.00 sec)

MySQL:

mysql> select DATE_ADD(a, INTERVAL 1 day ) from t ;
Field   1:  `DATE_ADD(a, INTERVAL 1 day )`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       STRING
Collation:  utf8mb4_0900_ai_ci (255)
Length:     116
Max_length: 10
Decimals:   31
Flags:


+------------------------------+
| DATE_ADD(a, INTERVAL 1 day ) |
+------------------------------+
| 2020-12-10                   |
+------------------------------+
1 row in set (0.01 sec)

[mengxin9014]

/run-check_dev_2

[mengxin9014]

/run-check_dev_2

[mengxin9014]

/run-check_dev_2

[mengxin9014]

/run-check_dev_2

[mengxin9014]

/run-check_dev_2