Clang: Add final C++20 flag for Clang 11.0+

-std=c++20 superseded -std=c++2a in upstream Clang commit
24ad121582454e625bdad125c90d9ac0dae948c8.

Modules/Compiler/Clang.cmake

@@ -146,7 +146,10 @@ macro(__compiler_clang_cxx_standards lang)
       set(CMAKE_${lang}17_EXTENSION_COMPILE_OPTION "-std=gnu++1z")
     endif()
 
-    if (NOT CMAKE_${lang}_COMPILER_VERSION VERSION_LESS "${_clang_version_std17}")
+    if(NOT CMAKE_${lang}_COMPILER_VERSION VERSION_LESS 11.0)
+      set(CMAKE_${lang}20_STANDARD_COMPILE_OPTION "-std=c++20")
+      set(CMAKE_${lang}20_EXTENSION_COMPILE_OPTION "-std=gnu++20")
+    elseif(NOT CMAKE_${lang}_COMPILER_VERSION VERSION_LESS "${_clang_version_std17}")
       set(CMAKE_${lang}20_STANDARD_COMPILE_OPTION "-std=c++2a")
       set(CMAKE_${lang}20_EXTENSION_COMPILE_OPTION "-std=gnu++2a")
     endif()