Add C++ 20 to CMake's language standard infrastructure (we already support 98, 11, 14, and 17). For now record only the
cxx_std_20 meta-feature. GNU and Clang compilers already support
-std=gnu++2a flags, so record these flags for relevant versions of those compilers. Also record MSVC support for
-std:c++latest on sufficient versions.
Fixes: #17849 (closed)