Add Clang 21 to CI

Author: ednolan

.github/workflows/ci_tests.yml

@@ -74,7 +74,7 @@ jobs:
                 }
               ]
             },
-            { "versions": ["20", "19"],
+            { "versions": ["21", "20", "19"],
               "tests": [
                 { "cxxversions": ["c++26", "c++23"],
                   "tests": [

README.md

@@ -132,6 +132,7 @@ This project officially supports:
 - GCC 15
 - Clang 19/libc++
 - Clang 20/libc++
+- Clang 21/libc++
 - MSVC 19.41
 
 Note: Building with Clang and libstdc++ is not currently supported due to various bugs.