Support C++20 feature testing

C++20 introduces a special predefined macro `__has_cpp_attribute()` along with a set of predefined language feature macros, all described [here](https://en.cppreference.com/w/cpp/feature_test). I think that at minimum `__has_cpp_attribute()` should be defined in C++20 mode, even if it always evaluates to `true`.

What's less clear to me is whether, and how, the return values should be user-programmable (e.g. for emulating a specific compiler version).