[libc++][CI] Add a Github workflow to test libc++ tools (#211067)

We have a few scripts under libcxx/utils, and they wouldn't be tested on
a regular basis. As a result, changes to e.g. the build system could
break these scripts, which in turn could break some Github workflows
like the libc++ PR benchmarking workflow.

This patch adds a new `test-tools` configuration to `run-buildbot` which
allows testing these tools locally, and a new dedicated Github workflow
to run it.

.github/workflows/libcxx-pr-test-tools.yml

@@ -0,0 +1,54 @@
+# This workflow exercises libc++ tools whose testing doesn't belong in the conformance
+# suite or in `check-cxx` proper.
+#
+# The actual logic lives in `libcxx/utils/ci/run-buildbot test-tools`, which is also how
+# failures can be reproduced locally (after installing libcxx/utils/requirements.txt):
+#
+#     CC=clang CXX=clang++ libcxx/utils/ci/run-buildbot test-tools
+
+name: "[libc++] Test tools"
+
+on:
+  pull_request:
+    paths:
+      - 'libcxx/**'
+      - 'libcxxabi/**'
+      - 'libunwind/**'
+      - 'runtimes/**'
+      - 'cmake/**'
+      - '.github/workflows/libcxx-pr-test-tools.yml'
+  schedule:
+    # Match the main libc++ CI cron
+    - cron: '0 8 * * *'
+
+permissions:
+  contents: read # Default everything to read-only
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test-tools:
+    if: github.repository_owner == 'llvm'
+    runs-on: llvm-premerge-libcxx-runners
+    env:
+      CC: clang-23
+      CXX: clang++-23
+    steps:
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+        with:
+          persist-credentials: false
+          fetch-depth: 0 # some tools need full git history
+          fetch-tags: true
+
+      - name: Set up Python dependencies
+        run: |
+          python3 -m venv .venv
+          source .venv/bin/activate
+          pip install -r libcxx/utils/requirements.txt
+
+      - name: Run the tooling smoke test
+        run: |
+          source .venv/bin/activate
+          libcxx/utils/ci/run-buildbot test-tools

libcxx/utils/ci/run-buildbot

@@ -270,6 +270,37 @@ check-generated-output)
            --exclude 'underflow.pass.cpp' \
            || false
 ;;
+test-tools)
+    #
+    # This job tests libc++ tools that don't make sense to be tested inside the
+    # libc++ conformance test suite.
+    #
+    clean
+
+    step "Building and installing libc++ with build-at-commit"
+    "${MONOREPO_ROOT}/libcxx/utils/build-at-commit" \
+        --git-repo "${MONOREPO_ROOT}" \
+        --commit HEAD \
+        --install-dir "${INSTALL_DIR}" \
+        --tmp-build-dir "${BUILD_DIR}/build-at-commit" \
+        -- -DCMAKE_CXX_COMPILER="${CXX}" \
+           -DCMAKE_BUILD_TYPE=RelWithDebInfo
+
+    step "Running a benchmark through the benchmark tooling pipeline"
+    "${MONOREPO_ROOT}/libcxx/utils/test-at-commit" \
+        --git-repo "${MONOREPO_ROOT}" \
+        --libcxx-installation "${INSTALL_DIR}" \
+        --build-dir "${BUILD_DIR}/benchmarks" \
+        --compiler "${CXX}"                   \
+        -- -sv --param optimization=speed \
+        "${MONOREPO_ROOT}/libcxx/test/benchmarks/hash.bench.cpp"
+
+    step "Consolidate benchmark results"
+    "${MONOREPO_ROOT}/libcxx/utils/consolidate-benchmarks" "${BUILD_DIR}/benchmarks" > "${BUILD_DIR}/results.lnt"
+
+    step "Compare benchmark results" # perform a no-op comparison, just to run the tool
+    "${MONOREPO_ROOT}/libcxx/utils/compare-benchmarks" "${BUILD_DIR}/results.lnt" "${BUILD_DIR}/results.lnt"
+;;
 #
 # Various Standard modes
 #