Revert "ci: remove clang-format testing"

This reverts commit d666a8d259.

Let's move past that tantrum and hope we can have an adult conversation
if one person wants to go against all the other developers.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23719>

.gitlab-ci/test/gitlab-ci.yml

@@ -30,6 +30,24 @@ rustfmt:
     - shopt -s globstar
     - rustfmt --check --verbose src/**/lib.rs
 
+clang-format:
+  # Cancel job if a newer commit is pushed to the same branch
+  interruptible: true
+  stage: lint
+  extends:
+    - .use-debian/x86_64_build
+    - .lint-clang-format-rules
+  variables:
+    GIT_STRATEGY: fetch
+    LLVM_VERSION: 15
+  script:
+    - shopt -s globstar
+    # We need a meson build dir, but its config doesn't actually matter, so
+    # let's just use the default.
+    - meson setup build
+    - ninja -C build clang-format
+    - git diff --color=always --exit-code  # Fails if there are diffs
+
 .test-gl:
   extends:
     - .test