libstdc++: Fix and complete __gnu_debug::basic_string implementation

libstdc++: Fix and complete __gnu_debug::basic_string implementation

Fix and complete __gnu_debug::basic_string so that it can be used as a transparent
replacement of std::basic_string.

libstdc++-v3/ChangeLog:

	* include/debug/string
	(basic_string(const basic_string&, const _Alloc&)): Define even if !_GLIBCXX_USE_CXX11_ABI.
	(basic_string(basic_string&&, const _Alloc&)): Likewise and add noexcept qualification.
	(basic_string<>::erase): Adapt to take __const_iterator.
	(basic_string(const _CharT*, const _Allocator&)): Remove assign call.
	(basic_string<>::insert(const_iterator, _InputIte, _InputIte)): Try to
	remove iterator debug layer even if !_GLIBCXX_USE_CXX11_ABI.
	[_GLIBCXX_USE_CHAR8_T] (__gnu_debug::u8string): New.
	(__gnu_debug::u16string, __gnu_debug::u32string): New.
	(std::hash<__gnu_debug::basic_string<>>): New partial specialization.
	(std::__is_fast_hash<__gnu_debug::basic_string<>>): Likewise.
	* testsuite/util/exception/safety.h
	(erase_base<__gnu_debug::basic_string<>>): New partial specialization.
	(insert_base<__gnu_debug::basic_string<>>): Likewise.
	* testsuite/util/testsuite_container_traits.h (traits<__gnu_debug::basic_string<>>):
	New partial specialization.
	* testsuite/21_strings/basic_string/hash/debug.cc: New test.
	* testsuite/21_strings/basic_string/requirements/citerators.cc:
	Add test on __gnu_debug::string.
	* testsuite/21_strings/basic_string/requirements/dr438/constructor.cc: Likewise.
	* testsuite/21_strings/basic_string/requirements/exception/basic.cc: Likewise.
	* testsuite/21_strings/basic_string/requirements/exception/generation_prohibited.cc:
	Likewise.
	* testsuite/21_strings/basic_string/requirements/exception/propagation_consistent.cc:
	Likewise.
	* testsuite/21_strings/basic_string/requirements/explicit_instantiation/char/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/requirements/explicit_instantiation/char16_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/requirements/explicit_instantiation/char32_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/requirements/explicit_instantiation/char8_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/requirements/explicit_instantiation/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/requirements/typedefs.cc: Likewise.