[WIP][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.

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

@@ -0,0 +1,29 @@
+name: "Check libc++ generated files"
+on:
+  pull_request_target:
+    paths:
+      - 'libcxx/**'
+permissions:
+  pull-requests: write
+
+jobs:
+  code_formatter:
+    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: seanmiddleditch/gha-setup-ninja@master
+        with:
+          destination: build/ninja-install
+
+      - name: Check generated files
+        run: libcxx/utils/ci/run-buildbot check-generated-output

libcxx/utils/ci/buildkite-pipeline.yml

@@ -33,24 +33,6 @@ 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: