Add GitHub Action to check that the generated source code is up to date

Author: ahoppen

.github/workflows/pull_request.yml

@@ -25,3 +25,15 @@ jobs:
       unacceptable_language_check_enabled: false
       # https://github.com/swiftlang/swift-syntax/issues/2991
       yamllint_check_enabled: false
+  verify_source_code:
+    name: Validate generated code
+    runs-on: ubuntu-latest
+    container:
+      image: swift:latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - name: Validate generated code
+        run: "./swift-syntax-dev-utils verify-source-code --toolchain /usr"