expression: Improve the compatibility of str_to_date (#25386)

[ti-srebot]

cherry-pick #25386 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25767

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git push git@github.com:ti-srebot/tidb.git pr/25767:release-5.0-8434069c5f63

---

What problem does this PR solve?

Issue Number: related to #24928

Problem Summary:

• '%b'/'%M' should be case insensitive
• Improve the compatibility of parsing some input for "%r" and "%T"

What is changed and how it works?

• Use strings.EqualFold for case insensitive prefix comparing in fullNameMonth
• Make abbreviatedMonth case insensitive
• Make time12Hour and time24Hour accept some irregular inputs

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Side effects

• Performance regression
  • Consumes more CPU
• Breaking backward compatibility

Release note

• Improve the MySQL compatibility of str_to_date for %b/%M/%r/%T

[ti-srebot]

/run-all-tests

[ti-srebot]

@JaySon-Huang you're already a collaborator in bot's repo.

[JaySon-Huang]

/run-unit-test

[JaySon-Huang]

/run-check_dev

[JaySon-Huang]

[2021-06-25T12:09:05.702Z] ./tools/check/check_testSuite.sh
[2021-06-25T12:09:07.604Z] tools/bin/golangci-lint run -v --disable-all --deadline=3m \
[2021-06-25T12:09:07.604Z]   --enable=misspell \
[2021-06-25T12:09:07.604Z]   --enable=ineffassign \
[2021-06-25T12:09:07.604Z]   --enable=varcheck \
...
[2021-06-25T12:09:19.085Z] level=info msg="[runner] linters took 5.641386383s with stages: goanalysis_metalinter: 5.626532342s"
[2021-06-25T12:09:19.085Z] types/time.go:3017:3: ineffectual assignment to `state` (ineffassign)
[2021-06-25T12:09:19.085Z] 		state := parseStateNormal
[2021-06-25T12:09:19.085Z] 		^
[2021-06-25T12:09:19.085Z] types/time.go:3095:3: ineffectual assignment to `state` (ineffassign)
[2021-06-25T12:09:19.085Z] 		state := parseStateNormal
[2021-06-25T12:09:19.085Z] 		^

Why this lint check didn't apply to the master branch?

[JaySon-Huang]

@wshwsh12 @guo-shaoge @wjhuang2016 PTAL

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lzmhhh123
• wshwsh12

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[zhouqiang-cl]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 2a33192

[ti-chi-bot]

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