libstdc++: Make string_view::copy usable in constant expressions (PR 94498)

libstdc++: Make string_view::copy usable in constant expressions (PR 94498)

	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.