Exclude LLVM 21 from Macos

Author: bolt12

.github/workflows/comprehensive-ci.yml

@@ -29,6 +29,11 @@ jobs:
         runner: [ubuntu-latest, macos-latest, windows-latest]
         ghc: ['9.2.8', '9.4.8', '9.6.7', '9.8.4', '9.10.3', '9.12.2']
         llvm: ['14', '16', '18', '20', '21']
+        exclude:
+          # LLVM 21 not available on macOS ARM64
+          - os: macos-latest
+            llvm-version: '21'
+
     steps:
       - name: Set git to use LF
         run: |