*: Fix use of user identity in SHOW GRANTS + error messages (#30294)

[ti-srebot]

cherry-pick #30294 to release-5.3
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/30474

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git push git@github.com:ti-srebot/tidb.git pr/30474:release-5.3-376d5428cca9

---

What problem does this PR solve?

Issue Number: fixes #30279 (once new tidb-test PR merged as well)

Problem Summary:

#30204 revealed a bug (i.e. it's not specifically a regression introduced) where the column used in show grants is from the Hostname of the user that logged in, not the matching entry from the MySQL privileges table.

Upon further inspection, it looks like we almost always intend to use the user from the privileges table and not the hostname of the logged in user. The exception is the USER() sql function.

(MySQL is actually inconsistent in which hostname it uses in error messages. The important case is with SHOW GRANTS FOR CURRENT_USER() which is used in integration tests.)

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

TiDB now more consistently prints the user+hostname of a user in error messages.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• dveeden
• tiancaiamao

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[ti-srebot]

/run-all-tests

[ti-srebot]

@morgo you're already a collaborator in bot's repo.

[guo-shaoge]

/run-mysql-test

[ti-chi-bot]

This pull request is closed because its related version has closed automatic cherry-picking.
If it's still needed, you can reopen it or just regenerate it using bot,
see:

https://prow.tidb.net/command-help#cherrypick
https://book.prow.tidb.net/#/plugins/cherrypicker