[libc++] Move the check-generated-files job to Github Actions

This allows running these quick checks faster than in our Buildkite
pipeline, which has much more latency. This will also avoid blocking
the rest of the testing pipeline in case the generated-files checks
are failing.

Author: ldionne

.github/workflows/libcxx-check-generated-files.yml

@@ -0,0 +1,25 @@
+name: "Check libc++ generated files"
+on:
+  pull_request:
+    paths:
+      - 'libcxx/**'
+
+jobs:
+  check_generated_files:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Fetch LLVM sources
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 2
+
+      - name: Install clang-format
+        uses: aminya/setup-cpp@v1
+        with:
+          clangformat: 17.0.1
+
+      - name: Install Ninja
+        uses: llvm/actions/install-ninja
+
+      - name: Check generated files
+        run: libcxx/utils/ci/run-buildbot check-generated-output

libcxx/utils/ci/buildkite-pipeline.yml

@@ -26,31 +26,12 @@ env:
     # LLVM POST-BRANCH bump version
     # LLVM POST-BRANCH add compiler test for ToT - 1, e.g. "Clang 17"
     # LLVM RELEASE bump remove compiler ToT - 3, e.g. "Clang 15"
-    LLVM_STABLE_VERSION: "17" # Used for tooling, update after the RELEASE.
     LLVM_HEAD_VERSION: "18"   # Used compiler, update POST-BRANCH.
     GCC_STABLE_VERSION: "13"
 steps:
   #
   # Light pre-commit tests for things like forgetting to update generated files.
   #
-  - label: "Generated output"
-    command: "libcxx/utils/ci/run-buildbot check-generated-output"
-    artifact_paths:
-      - "**/generated_output.patch"
-      - "**/generated_output.status"
-    env:
-        CC: "clang-${LLVM_HEAD_VERSION}"
-        CXX: "clang++-${LLVM_HEAD_VERSION}"
-        CLANG_FORMAT: "/usr/bin/clang-format-${LLVM_STABLE_VERSION}"
-    agents:
-      queue: "libcxx-builders"
-      os: "linux"
-    retry:
-      automatic:
-        - exit_status: -1  # Agent was lost
-          limit: 2
-    timeout_in_minutes: 120
-
   - label: "Documentation"
     command: "libcxx/utils/ci/run-buildbot documentation"
     artifact_paths: