[clang] [emacs][clang-format] Add elisp API for clang-format on git diffs (PR #112792)

[via cfe-commits]

goldsteinn wrote:

> Note that I'm not a regular clang contributor, I just submitted a small improvement to `clang-format.el`, and maintain some emacs packages in elpa & melpa.
> 
> Overall the PR looks like it needs more attention to detail, as far as I can tell it's creating temporary files and never removing them, various minor issues noted inline.
> 
>     * This PR needs to be rebased on top of the recently added `clang-format-on-save-mode` commit.
> 
>     * Running `package-lint` reports.
> 
> 
> ```
> 156:19: warning: Closing parens should not be wrapped onto new lines.
> 157:18: warning: Closing parens should not be wrapped onto new lines.
> 176:12: error: You should depend on (emacs "24.4") or the compat package if you need `string-trim'.
> 188:11: error: You should depend on (emacs "25.1") if you need `vc-root-dir'.
> 198:59: error: You should depend on (emacs "25.1") if you need `vc-root-dir'.
> ```

Thank you for the detailed feedback, I will have v2 up tomorrow.

Regarding the package lints, is requiring 25.1 acceptable? I'm not really sure how to implement this without `vc-root-dir`.

https://github.com/llvm/llvm-project/pull/112792