libstdc++: Implement constexpr std::basic_string for C++20

This is only supported for the cxx11 ABI, not for COW strings.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string, operator""s): Add
	constexpr for C++20.
	(basic_string::basic_string(basic_string&&)): Only copy
	initialized portion of the buffer.
	(basic_string::basic_string(basic_string&&, const Alloc&)):
	Likewise.
	* include/bits/basic_string.tcc (basic_string): Add constexpr
	for C++20.
	(basic_string::swap(basic_string&)): Only copy initialized
	portions of the buffers.
	(basic_string::_M_replace): Add constexpr implementation that
	doesn't depend on pointer comparisons.
	* include/bits/cow_string.h: Adjust comment.
	* include/ext/type_traits.h (__is_null_pointer): Add constexpr.
	* include/std/string (erase, erase_if): Add constexpr.
	* include/std/version (__cpp_lib_constexpr_string): Update
	value.
	* testsuite/21_strings/basic_string/cons/char/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/literals/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/modifiers/constexpr.cc: New test.
	* testsuite/21_strings/basic_string/modifiers/swap/char/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/modifiers/swap/wchar_t/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/version.cc: New test.