PR libstdc++/94498
* include/bits/char_traits.h (__gnu_cxx::char_traits::move): Make it
usable in constant expressions for C++20.
(__gnu_cxx::char_traits::copy, __gnu_cxx::char_traits::assign): Add
_GLIBCXX20_CONSTEXPR.
(std::char_traits<char>, std::char_traits<wchar_t>)
(std::char_traits<char8_t>): Make move, copy and assign usable in
constant expressions for C++20.
(std::char_traits<char16_t>, std::char_traits<char32_t>): Make move
and copy usable in constant expressions for C++20.
* include/std/string_view (basic_string_view::copy): Add
_GLIBCXX20_CONSTEXPR.
* testsuite/21_strings/basic_string_view/operations/copy/char/
constexpr.cc: New test.
* testsuite/21_strings/basic_string_view/operations/copy/wchar_t/
constexpr.cc: New test.
