[llvm] [CI] Refactor common functionality into separate script (PR #119530)

[David Spickett via llvm-commits]

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

Seems like a simple move, nothing controversial.

Any big changes to the code are best done when the new system is stable anyway.

https://github.com/llvm/llvm-project/pull/119530