[libc++] mark std::expected as nodiscard (fix #130656)

Author: hulxv

libcxx/include/__cxx03/__expected/expected.h

@@ -58,7 +58,7 @@ _LIBCPP_PUSH_MACROS
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp, class _Err>
-class expected;
+class [[nodiscrad]] expected;
 
 template <class _Tp>
 struct __is_std_expected : false_type {};

libcxx/include/__expected/expected.h

@@ -60,7 +60,7 @@ _LIBCPP_PUSH_MACROS
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp, class _Err>
-class expected;
+class [[nodiscard]] expected;
 
 template <class _Tp>
 struct __is_std_expected : false_type {};