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>