Publish LLVM's buildbot fork under llvm GitHub org

[asb]

One of the limitations with the current "local testing" mode of llvm-zorg, as also used in pre-commit tests via GitHub Actions, is that we can't use the same buildbot code deployed to lab.llvm.org. A buildbot fork based on 3.11.7 but with downstream patches seems to be used, but I can't see it published anywhere (requirements.in just references a local checkout). Having this fork published somewhere (ideally under the llvm GitHub org), would mean I can move to using that in pre-commit tests, avoiding issues like #323. See also: discussion in #329 and #325.

CC @nickdesaulniers

[nickdesaulniers]

Or having an upstream first policy wrt buildbot changes.