Switch back to older LLVM version.

Author: breyerml

.github/workflows/clang_macos.yml

@@ -17,7 +17,7 @@ jobs:
       - name: "Install LLVM and Clang"
         uses: KyleMayes/install-llvm-action@v2
         with:
-          version: "18.1.8"
+          version: "15.0"
       - name: "Install dependencies via brew"
         run: |
           brew install libomp