Update LLVM workflow to latest Action images.

Author: tritao

.github/workflows/llvm.yml

@@ -9,9 +9,9 @@ jobs:
       fail-fast: false
       matrix:
         config:
-          - { os: ubuntu-20.04, platform: x64, cxx: g++-9, cc: gcc-9 }
-          - { os: ubuntu-20.04, platform: x64, cxx: g++-10, cc: gcc-10 }
-          - { os: macos-10.15, platform: x64, cxx: clang++, cc: clang }
+          - { os: ubuntu-22.04, platform: x64, cxx: g++-11, cc: gcc-11 }
+          - { os: macos-11, platform: x64, cxx: clang++, cc: clang }
+          - { os: macos-12, platform: x64, cxx: clang++, cc: clang }
 
     runs-on: ${{ matrix.config.os }}