Add /std:c++20 test configurations #1658
Description
In VS 2019 16.10 Preview 2, the compiler will be adding a /std:c++20 switch (in addition to /std:c++latest). We'll need to add test configurations and matrices so we can continue to test C++20 specifically, even as the compiler begins to implement C++23 features in the future. (Note: We are still focusing on C++20 in the library and are not yet implementing C++23 features.)