[llvm] Partially Reapply "[libcxx] Move premerge jobs to k8s mode runner set (#221395)" (PR #225222)

[Louis Dionne via llvm-commits]

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

LGTM, thanks for investigating. However, since this touches benchmark runners, I think we should also discuss https://github.com/llvm/llvm-project/issues/225586 in the near future since IDK if the change to K8s basically made our benchmarking jobs meaningless (or perhaps not).

https://github.com/llvm/llvm-project/pull/225222