[llvm] Reapply [workflows] Split pr-code-format into two parts to make it more secure (#78215) (PR #80495)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> @jyknight @boomanaiden154 So it's OK to commit this as-is and then we switch to using the merge commit in a follow up patch?

I think it should be good. I would think any work being done to move this to checkout the default `pull_request` ref would either need to be done in this patch, or would be preferred to be done after this patch, so it should just land to make forward progress there.

This just keeps the current state of things and doesn't introduce any regressions while being more secure, other than the (somewhat) edge case where there are merge conflicts, so seems fine to land to me.

https://github.com/llvm/llvm-project/pull/80495