CppTools: Code model fix for C++20, MSVC, and newer CMake versions

Starting with version 3.20 CMake adds -std:c++20, which breaks the
code model when using MSVC.

Clang-cl 12 doesn't know about -std:c++20, but clang driver knows
about -std=c++20.

Fixes: QTCREATORBUG-26146
Change-Id: I696842e11b0a9ba8849455d2f81f8dde6dd95a27
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/libs/utils/cpplanguage_details.h

@@ -43,8 +43,9 @@ enum class LanguageVersion : unsigned char {
     CXX11,
     CXX14,
     CXX17,
-    CXX2a,
-    LatestCxx = CXX2a,
+    CXX20,
+    CXX2b,
+    LatestCxx = CXX2b,
 };
 
 enum class LanguageExtension : unsigned char {