ci: add llvm18 to ci matrix

Author: cpunion

.github/workflows/go.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         os: [macos-latest, ubuntu-latest]
-        llvm: [17]
+        llvm: [17, 18]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4