Use __cpp_nontype_template_args instead of __cpp_nontype_template_parameter_class

[paul082]

Appears only GCC defines __cpp_nontype_template_parameter_class.

Also Cppreference makes no mention of a __cpp_nontype_template_parameter_class macro. https://en.cppreference.com/w/cpp/feature_test

[foonathan]

It's a bit confusing, thanks for catching it. https://wg21.link/SD6 lists both as feature check macros, although I believe the feature from __cpp_nontype_template_parameter_class is no longer part of C++20. Anyways, I'll just check for both.