Pre-merge testing and transition to PRs

With @ldionne (thak you Louis!) we have made a number of changes to pre-merge check preparing to transition from Phabricator to PRs.

The intent is to support both Phabricator and PRs with the same level of testing until October 1. After that current repo llvm-premerge-checks will likely be archived or will contain only container images and other “boring” parts.

The logic is now located in llvm project .ci/generate-buildkite-pipeline-premerge. If your project would benefit from a custom workflow please follow the example of libcxx or just modify cmake flags for your project.

Naturally there will some falures due to the changes. For example, for Phabricator reviews that are based on the old commits the build might not run or not report results back. Rebasing at HEAD should fix that.

Please reach #infrastructure at Discord if something is broken, especially if it affects Pull Requests.