Add Clang compilation CI job on macOS

Add a clang_compile job to verify that the C extension compiles
cleanly with Clang, enabling nullability checks via
-Wnullable-to-nonnull-conversion. The job runs on macOS where
Ruby is built with Apple Clang, so mkmf's flag checks use the
correct compiler.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: soutaro

.github/workflows/ruby.yml

@@ -110,3 +110,37 @@ jobs:
         run: |
           bin/setup
       - run: bundle exec rake clean compile_c99
+
+  clang_compile:
+    runs-on: macos-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: ['4.0', head]
+    steps:
+      - uses: actions/checkout@v6
+      - name: Install dependencies
+        run: |
+          brew install ruby-build
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler: none
+      - name: Set working directory as safe
+        run: git config --global --add safe.directory $(pwd)
+      - name: Update rubygems & bundler
+        run: |
+          ruby -v
+          gem update --system
+      - name: install erb
+        run: gem install erb
+      - name: clang version
+        run: clang --version
+      - name: bundle config set force_ruby_platform true if head
+        if: ${{ contains(matrix.ruby, 'head') }}
+        run: |
+          bundle config set force_ruby_platform true
+      - name: bin/setup
+        run: |
+          bin/setup
+      - run: bundle exec rake clean compile