[clang] Move the Clang CI jobs off of the libc++ builders

[ldionne]

Summary

The libc++ builders do not have the capacity to serve both libc++ and
Clang CI needs. We initially used the libc++ builders when adding
pre-commit CI for Clang as an experiment, however this setup has been
problematic for libc++ for a long time. It makes libc++ CI turnaround
way too slow, which has been a frequent source of frustration.

We didn't want to make this change before the release to avoid rocking
the boat too much in such an important time, however now that the
release has been cut, we need to make this change. This will shift
the traffic for Clang pre-commit CI from the libc++ builders to the
builders that have been used for Clang pre-commit CI defined outside
of Clang's BuildKite pipeline.

[ldionne]

@goncharov Now that we have a buildkite-pipeline.yml for Clang, would it make sense to stop running the "builtin" jobs like https://buildkite.com/llvm-project/premerge-checks/builds/169498#0189d1af-3c81-4ab5-a87a-ac8db177ccf0 and instead run these tests inside the Clang CI pipeline by defining them inside buildkite-pipeline.yml? This would centralize where all the jobs for Clang CI are defined.

[ldionne]

Try to fix build on queue=linux builders

[aaron.ballman]

I'm not qualified to give a review of the functional changes, but the intent for the changes SGTM!

[philnik]

Thanks for working on this! I think it would make a lot of sense to unify this build and the Debian build, since currently we are building Clang twice without a good reason.

[Mordante]

Nice to see the Clang CI new builder! Thanks for working on this, LGTM!

clang/utils/ci/buildkite-pipeline.yml

env:
     # LLVM RELEASE bump version
     LLVM_HEAD_VERSION: "17"
 steps:
   - label: "Format"
     commands:
       - "clang/utils/ci/run-buildbot check-format"
 
     agents:
-      queue: "libcxx-builders"
-      os: "linux"
+      queue: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120
 
   - wait
 
   - label: "Building clang"
     commands:
       - "clang/utils/ci/run-buildbot build-clang"
-    env:
-        CC: "clang-${LLVM_HEAD_VERSION}"
-        CXX: "clang++-${LLVM_HEAD_VERSION}"
     agents:
-      queue: "libcxx-builders"
-      os: "linux"
+      queue: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120
 
   - wait
 
-  - label: "C++03"
+  - label: "Running libc++ test suite in C++03"
     commands:
       - "clang/utils/ci/run-buildbot generic-cxx03"
     artifact_paths:
       - "**/test-results.xml"
       - "**/crash_diagnostics/*"
     env:
         LLVM_SYMBOLIZER_PATH: "/usr/bin/llvm-symbolizer-${LLVM_HEAD_VERSION}" # TODO: Should we build that from scratch?
         CLANG_CRASH_DIAGNOSTICS_DIR: "crash_diagnostics"
     agents:
-      queue: "libcxx-builders"
-      os: "linux"
+      queue: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120
 
-  - label: "C++26"
+  - label: "Running libc++ test suite in C++26"
     commands:
       - "clang/utils/ci/run-buildbot generic-cxx26"
     artifact_paths:
       - "**/test-results.xml"
       - "**/crash_diagnostics/*"
     env:
         LLVM_SYMBOLIZER_PATH: "/usr/bin/llvm-symbolizer-${LLVM_HEAD_VERSION}" # TODO: Should we build that from scratch?
         CLANG_CRASH_DIAGNOSTICS_DIR: "crash_diagnostics"
     agents:
-      queue: "libcxx-builders"
-      os: "linux"
+      queue: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120
 
-  - label: "Modules"
+  - label: "Running libc++ test suite with Clang Modules"
     commands:
       - "clang/utils/ci/run-buildbot generic-modules"
     artifact_paths:
       - "**/test-results.xml"
       - "**/crash_diagnostics/*"
     env:
         LLVM_SYMBOLIZER_PATH: "/usr/bin/llvm-symbolizer-${LLVM_HEAD_VERSION}" # TODO: Should we build that from scratch?
         CLANG_CRASH_DIAGNOSTICS_DIR: "crash_diagnostics"
     agents:
-      queue: "libcxx-builders"
-      os: "linux"
+      queue: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120