[libc++] [CI] Validate the output of the generated scripts.

This adds a CI job validating that the output of
utils/generate_feature_test_macro_components.py,
libcxx/utils/generate_header_inclusion_tests.py, and
utils/generate_header_tests.py are up to date.

The validation method has been copied from the Format job.

Differential Revision: https://reviews.llvm.org/D99862

libcxx/utils/ci/buildkite-pipeline.yml

@@ -28,6 +28,24 @@ steps:
     soft_fail:
         - exit_status: 1
 
+  - label: "Generated output"
+    command: "libcxx/utils/ci/run-buildbot check-generated-output"
+    artifact_paths:
+      - "**/generated_output.patch"
+    agents:
+      queue: "libcxx-builders"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+
+  # All jobs defined before this `wait` will run whenever a CI job is started.
+  # Jobs defined after the `wait` will run only if all the jobs above
+  # succeeded. The build failures before this `wait` can be fixed by running
+  # the proper scripts or clang-format. We use this to reduce the load on the
+  # testers and avoid running builds that we know fail anyway.
+  - wait
+
   - label: "C++03"
     command: "libcxx/utils/ci/run-buildbot generic-cxx03"
     artifact_paths:

libcxx/utils/ci/run-buildbot

@@ -132,6 +132,17 @@ check-format)
     # Check if the diff is empty, fail otherwise.
     ! grep -q '^--- a' ${BUILD_DIR}/clang-format.patch
 ;;
+check-generated-output)
+    clean
+    echo "+++ Checking the output of the generated scripts"
+    mkdir -p ${BUILD_DIR}
+    python3 libcxx/utils/generate_feature_test_macro_components.py
+    python3 libcxx/utils/generate_header_inclusion_tests.py
+    python3 libcxx/utils/generate_header_tests.py
+    git diff | tee ${BUILD_DIR}/generated_output.patch
+    # Check if the diffs are empty, fail otherwise.
+    ! grep -q '^--- a' ${BUILD_DIR}/generated_output.patch
+;;
 generic-cxx03)
     export CC=clang
     export CXX=clang++