[llvm] [CI] Unify handling of scripted PR workflows (PR #199127)

[Oliver Hunt via llvm-commits]

ojhunt wrote:

> Can you link actions that are already doing this? I'm skeptical a new composite action actually makes things simpler.
> 
> I'm also not sure how much this actually helps. Premerge is going to have way more problems than code formatting/linting since it just runs the build, and has the same exposure as formatting/linting.

do you mean the "Oliver read about composites and tried to write one?" or the hackery around getting the baseline? the latter was based on google and trying to get past the endless llm generated garbage that now fills the world.

I wasn't sure if it was correct to do the sparse checkout things like this, because they all seemed to occur in gated workflows.

```yaml
      - name: Checkout LLVM
        uses: actions/checkout at de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
        with:
          persist-credentials: false
          sparse-checkout: |
            .github/workflows/containers/github-action-ci-tooling/
            ...
```

The need for a separate checkout seemed plausible, but there's also a reason that I mass added CI folk to this PR :D

https://github.com/llvm/llvm-project/pull/199127