[libcxx] add [[nodiscard]] to expected.h

Author: hulxv

libcxx/include/__expected/expected.h

@@ -1354,7 +1354,7 @@ class __expected_void_base {
 
 template <class _Tp, class _Err>
   requires is_void_v<_Tp>
-class expected<_Tp, _Err> : private __expected_void_base<_Err> {
+class [[nodiscard]] expected<_Tp, _Err> : private __expected_void_base<_Err> {
   static_assert(__valid_std_unexpected<_Err>::value,
                 "[expected.void.general] A program that instantiates expected<T, E> with a E that is not a "
                 "valid argument for unexpected<E> is ill-formed");