[libcxx] [llvm] Add pre-commit hooks and git config for libc++ & git-clang-format. (PR #73798)

[Louis Dionne via llvm-commits]

ldionne wrote:

@EricWF Gentle ping -- do you still want to move forward with this or should we close the PR?

IMO we could take the documentation you wrote and move it to be LLVM-wide with minor changes.

https://github.com/llvm/llvm-project/pull/73798