[llvm] code-format: Improve the code-format-helper to be able to run as a git hook (PR #73957)

[Louis Dionne via llvm-commits]

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@@ -14,8 +14,22 @@
 import sys
 from functools import cached_property
 
-import github
-from github import IssueComment, PullRequest
+
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ldionne wrote:

Can some documentation be added somewhere explaining what this script does and in particular what it can be used as a pre-commit hook (and how to install it)? Since we expect a lot of people to start installing this as a hook, it makes sense to properly document this script. I'm not saying the details of how it gets called in the pre-commit CI and whatnot, but at least
- it runs both `clang-format` and `darker`
- it is the canonical way of formatting stuff in LLVM
- it can be installed as a pre-commit hook (and how)
- what "customization points" exist (e.g. the environment variables to pick up Darker and clang-format)


https://github.com/llvm/llvm-project/pull/73957