Add step to install Clang21 to CI script

Author: andreasbuhr

.github/workflows/cmake.yml

@@ -514,6 +514,17 @@ jobs:
         sudo ./llvm.sh 20
         sudo apt-get install libc++-20-dev libc++abi-20-dev libunwind-20-dev
 
+    - name: Install Clang 21
+      id: install_clang_21
+      if: startsWith(matrix.config.os, 'ubuntu') && ( matrix.config.cxx == 'clang++-21' )
+      shell: bash
+      working-directory: ${{ env.HOME }}
+      run: |
+        wget https://apt.llvm.org/llvm.sh
+        chmod +x llvm.sh
+        sudo ./llvm.sh 21
+        sudo apt-get install libc++-21-dev libc++abi-21-dev libunwind-21-dev
+
     - name: Install g++ 10
       id: install_gcc_10
       if: startsWith(matrix.config.os, 'ubuntu') && ( matrix.config.cxx == 'g++-10' )