<filesystem>: Set the error code for fs::absolute("", ec).

[leejy12]

Closes #6120

fs::absolute("", ec) was returning an empty path, indicating an error has occurred, but didn't set ec to the appropriate error_code.

libstdc++ treats this call as an error, whereas libc++ treats it as a success case.

[fs.op.absolute#1] says the function "Composes an absolute path referencing the same file system location as p according to the operating system". On Windows, GetFullPathNameW(L"", ...) fails, so following libstdc++’s behavior is more consistent with the standard wording.

This change updates the implementation to set ec.value() to 123 (ERROR_INVALID_NAME).

[YexuanXiao]

The error code for calling CreateFile with an empty string is ERROR_PATH_NOT_FOUND (3). Is it more appropriate to use it? This means the result obtained when not converting it to an absolute path.

[leejy12]

@YexuanXiao I intended to let __std_fs_get_full_path_name populate it automatically.

STL/stl/src/filesystem.cpp

Lines 156 to 160 in 117ca96

	[[nodiscard]] __std_ulong_and_error __stdcall __std_fs_get_full_path_name(_In_z_ const wchar_t* _Source,
	_In_ unsigned long _Target_size, _Out_writes_z_(_Target_size) wchar_t* _Target) noexcept { // calls GetFullPathNameW
	const auto _Result = GetFullPathNameW(_Source, _Target_size, _Target, nullptr);
	return {_Result, _Result == 0 ? __std_win_error{GetLastError()} : __std_win_error::_Success};
	}

But libstdc++ sets the error to EINVAL (22), so a better error may be ERROR_INVALID_PARAMETER (87). I think if we just return this when _Input._Text.empty() is true, we can avoid calling the Win32 API.