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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

This is mainly to enable setting up Github Actions Workflows using the existing CI scripts (like https://github.com/llvm/llvm-project/pull/119635).

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