[llvm] workflows: Merge LLVM tests together into a single job (PR #78877)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM.

This is a nice simplification. I'm assuming since this goes to MacOS13, we don't end up hitting https://github.com/llvm/llvm-project/pull/77270?

https://github.com/llvm/llvm-project/pull/78877