[llvm] workflows: Add a simple pull request subscription workflow (PR #64913)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Thank you for creating this patch and starting https://discourse.llvm.org/t/changes-to-pull-request-subscription-system/73296  . I only got to understand how this patch will help the situation after reading the Discourse post. This looks great to replace the current pr-subscribers via `.github/CODEOWNERS` mechanism.

https://github.com/llvm/llvm-project/pull/64913