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

[Campbell Barton via cfe-commits]

https://github.com/ideasman42 requested changes to this pull request.

Note that I'm not a regular clang contributor, I just submitted a small improvement to clang-format, 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'.
```

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