C++20 constexpr _Copy_s and simplify char8_t compare

[StephanTLavavej]

Followup to #491, in response to #491 (comment) and #491 (comment).

• Mark our non-Standard extension char_traits::_Copy_s and basic_string_view::_Copy_s as _CONSTEXPR20. Similarly to how Misc constexpr #491 changed copy, previously _Copy_s was always constexpr in basic_string_view but never in char_traits; I similarly argue that this will not affect users because it would have required them to notice and provide a custom traits class (and in this case, the non-Standard extension is far more obscure than the Standard function). Making these constexpr in C++20 mode only is simpler to explain.

• Simplify char_traits::compare for char8_t. This is possible because memcmp and __builtin_memcmp take const void* and return int, and their behavior is correct for char8_t. The other u8 intrinsics are type-sensitive so they can't be unified like this.

[StephanTLavavej]

Thanks, @CaseyCarter, for these improvement ideas!