Fix several bugs in the encoding conversions for filesystem::path that
prevent conversion of Unicode characters outside the Basic Multilingual
Plane, and prevent returning basic_string specializations with
alternative allocator types.

The std::codecvt_utf8 class template is not suitable for UTF-16
conversions because it uses UCS-2 instead. For conversions between UTF-8
and UTF-16 either std::codecvt<C, char, mbstate> or
codecvt_utf8_utf16<C> must be used.

The __str_codecvt_in and __str_codecvt_out utilities do not
return false on a partial conversion (e.g. for invalid or incomplete
Unicode input). Add new helpers that treat partial conversions as
errors, and use them for all filesystem::path conversions.

	PR libstdc++/90281 Fix string conversions for filesystem::path
	* include/bits/fs_path.h (u8path) [_GLIBCXX_FILESYSTEM_IS_WINDOWS]:
	Use codecvt_utf8_utf16 instead of codecvt_utf8. Use
	__str_codecvt_in_all to fail for partial conversions and throw on
	error.
	[!_GLIBCXX_FILESYSTEM_IS_WINDOWS && _GLIBCXX_USE_CHAR8_T]
	(path::_Cvt<char8_t>): Add explicit specialization.
	[_GLIBCXX_FILESYSTEM_IS_WINDOWS] (path::_Cvt::_S_wconvert): Remove
	overloads.
	[_GLIBCXX_FILESYSTEM_IS_WINDOWS] (path::_Cvt::_S_convert): Use
	if-constexpr instead of dispatching to _S_wconvert. Use codecvt
	instead of codecvt_utf8. Use __str_codecvt_in_all and
	__str_codecvt_out_all.
	[!_GLIBCXX_FILESYSTEM_IS_WINDOWS] (path::_Cvt::_S_convert): Use
	codecvt instead of codecvt_utf8. Use __str_codecvt_out_all.
	(path::_S_str_convert) [_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Use
	codecvt_utf8_utf16 instead of codecvt_utf8. Construct return values
	with allocator. Use __str_codecvt_out_all. Fallthrough to POSIX code
	after converting to UTF-8.
	(path::_S_str_convert): Use codecvt instead of codecvt_utf8. Use
	__str_codecvt_in_all.
	(path::string): Fix initialization of string types with different
	allocators.
	(path::u8string) [_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Use
	codecvt_utf8_utf16 instead of codecvt_utf8. Use __str_codecvt_out_all.
	* include/bits/locale_conv.h (__do_str_codecvt): Reorder static and
	runtime conditions.
	(__str_codecvt_out_all, __str_codecvt_in_all): New functions that
	return false for partial conversions.
	* include/experimental/bits/fs_path.h (u8path):
	[_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Implement correctly for mingw.
	[_GLIBCXX_FILESYSTEM_IS_WINDOWS] (path::_Cvt::_S_wconvert): Add
	missing handling for char8_t. Use codecvt and codecvt_utf8_utf16
	instead of codecvt_utf8. Use __str_codecvt_in_all and
	__str_codecvt_out_all.
	[!_GLIBCXX_FILESYSTEM_IS_WINDOWS] (path::_Cvt::_S_convert): Use
	codecvt instead of codecvt_utf8. Use __str_codecvt_out_all.
	(path::string) [_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Use
	codecvt_utf8_utf16 instead of codecvt_utf8. Construct return values
	with allocator. Use __str_codecvt_out_all and __str_codecvt_in_all.
	(path::string) [!_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Use
	__str_codecvt_in_all.
	(path::u8string) [_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Use
	codecvt_utf8_utf16 instead of codecvt_utf8. Use __str_codecvt_out_all.
	* src/c++17/fs_path.cc (path::_S_convert_loc): Use
	__str_codecvt_in_all.
	* src/filesystem/path.cc (path::_S_convert_loc): Likewise.
	* testsuite/27_io/filesystem/path/construct/90281.cc: New test.
	* testsuite/27_io/filesystem/path/factory/u8path.cc: New test.
	* testsuite/27_io/filesystem/path/native/string.cc: Test with empty
	strings and with Unicode characters outside the basic multilingual
	plane.
	* testsuite/27_io/filesystem/path/native/alloc.cc: New test.
	* testsuite/experimental/filesystem/path/construct/90281.cc: New test.
	* testsuite/experimental/filesystem/path/factory/u8path.cc: New test.
	* testsuite/experimental/filesystem/path/native/alloc.cc: New test.
	* testsuite/experimental/filesystem/path/native/string.cc: Test with
	empty strings and with Unicode characters outside the basic
	multilingual plane.

From-SVN: r272385