try to add clang-19 to CI

.github/workflows/linux.yml

@@ -83,6 +83,20 @@ jobs:
             with_tests: ON
             cmake_options: -DQUILL_SANITIZE_THREAD=ON
 
+            # Build with modern clang version
+          - cxx: clang++-19
+            build_type: Release
+            std: 17
+            os: ubuntu-20.04
+            with_tests: ON
+            install: |
+              wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+              sudo add-apt-repository "deb http://apt.llvm.org/focal/ llvm-toolchain-focal-19 main"
+              sudo apt-get update
+              for i in {1..3}; do
+                sudo apt-get -o Acquire::Retries=3 install -y clang-19 libc++-19-dev libc++abi-19-dev && break || sleep 15
+              done
+
     steps:
       - uses: actions/checkout@v4
 
@@ -95,6 +109,10 @@ jobs:
         run: |
           sudo apt-get update
           ${{matrix.install}}
+          if [[ "${{matrix.cxx}}" == "clang++-19" ]]; then
+            sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-19 100
+            sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-19 100
+          fi
           cmake -E make_directory ${{runner.workspace}}/build
 
       - name: Configure