Add clang-21 to the matrix

New dev branch of clang exists, add it to the mix of compilers.

Author: steve-downey

.github/workflows/ci.yml

@@ -13,6 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         config:
+          - {name: "Ubuntu Clang 21", os: ubuntu-24.04, toolchain: "clang-21", clang_version: 21, installed_clang_version: 17, cmake_args: "-G \"Ninja Multi-Config\" -DCMAKE_CONFIGURATION_TYPES=\"RelWithDebInfo;Asan\" "}
           - {name: "Ubuntu Clang 20", os: ubuntu-24.04, toolchain: "clang-20", clang_version: 20, installed_clang_version: 17, cmake_args: "-G \"Ninja Multi-Config\" -DCMAKE_CONFIGURATION_TYPES=\"RelWithDebInfo;Asan\" "}
           # Note: clang-19 + Asan setup causes errors on some platforms. Temporary skip some checks via .asan_options.
           - {name: "Ubuntu Clang 19", os: ubuntu-24.04, toolchain: "clang-19", clang_version: 19, installed_clang_version: 17, cmake_args: "-G \"Ninja Multi-Config\" -DCMAKE_CONFIGURATION_TYPES=\"RelWithDebInfo;Asan\" ", asan_options: "new_delete_type_mismatch=0"}

etc/clang-21-toolchain.cmake

@@ -0,0 +1,11 @@
+# cmake-format: off
+# etc/clang-20-toolchain.cmake -*-cmake-*-
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+# cmake-format: on
+
+include_guard(GLOBAL)
+
+set(CMAKE_C_COMPILER clang-21)
+set(CMAKE_CXX_COMPILER clang++-21)
+
+include("${CMAKE_CURRENT_LIST_DIR}/clang-flags.cmake")