ci: LLVM install falls back to binaries

.github/workflows/ci.yml

@@ -177,6 +177,7 @@ jobs:
         id: llvm-parameters
         run: |
           echo -E "llvm-hash=7a28a5b3fee6c78ad59af79a3d03c00db153c49f" >> $GITHUB_OUTPUT
+          echo -E "llvm-id=7a28a5b3" >> $GITHUB_OUTPUT
           echo -E "llvm-build-preset=${{ runner.os == 'Windows' && 'release-win' || 'release-unix' }}" >> $GITHUB_OUTPUT
           cd ..
           llvm_root=$(pwd)/third-party/llvm-project/install
@@ -227,7 +228,29 @@ jobs:
               cmake --build ./build --target help
           fi          
           N_CORES=$(nproc 2>/dev/null || echo 1)
-          cmake --build ./build --config Release --parallel $N_CORES 
+          if [ ${{ runner.os }} != 'Windows' ]; then
+              cmake --build ./build --config Release --parallel $N_CORES
+          else
+              # Allow build step to fail
+              set +e
+              cmake --build ./build --config Release --parallel $N_CORES
+              exit_code=$?
+              set -e
+              if [ $exit_code -ne 0 ]; then
+                  echo "Could not build LLVM in CI. Fetching pre-built binaries from mrdocs.com."
+                  cd ..
+                  llvm_archive_filename="Windows-Release-${{ steps.llvm-parameters.outputs.llvm-id }}.7z"
+                  llvm_url="https://mrdocs.com/llvm+clang/$llvm_archive_filename"
+                  curl -L -o "$llvm_archive_filename" "$llvm_url"
+                  7z x "$llvm_archive_filename"
+                  cd "Release"
+                  mkdir -p "$llvm_project_root"/install
+                  mv * "$llvm_project_root"/install
+                  cd ..
+                  rm -rf "Release"
+                  exit 0
+              fi
+          fi
           cmake --install ./build --prefix "$llvm_project_root"/install
 
       - name: Install Node.js