[libc++] LWG4025: Move assignment operator of std::expected<cv void, E> should not be conditionally deleted (#109363)

This patch implements LWG4025: Move assignment operator of
`std::expected<cv void, E>` should not be conditionally deleted.

Closes #105324

Author: Xiaoyang Liu

libcxx/docs/Status/Cxx2cIssues.csv

@@ -50,7 +50,7 @@
 "`LWG4013 <https://wg21.link/LWG4013>`__","``lazy_split_view::outer-iterator::value_type`` should not provide default constructor","2024-03 (Tokyo)","","",""
 "`LWG4016 <https://wg21.link/LWG4016>`__","container-insertable checks do not match what container-inserter does","2024-03 (Tokyo)","","",""
 "`LWG4023 <https://wg21.link/LWG4023>`__","Preconditions of ``std::basic_streambuf::setg/setp``","2024-03 (Tokyo)","|Complete|","19.0",""
-"`LWG4025 <https://wg21.link/LWG4025>`__","Move assignment operator of ``std::expected<cv void, E>`` should not be conditionally deleted","2024-03 (Tokyo)","","",""
+"`LWG4025 <https://wg21.link/LWG4025>`__","Move assignment operator of ``std::expected<cv void, E>`` should not be conditionally deleted","2024-03 (Tokyo)","|Complete|","20.0",""
 "`LWG4030 <https://wg21.link/LWG4030>`__","Clarify whether arithmetic expressions in ``[numeric.sat.func]`` are mathematical or C++","2024-03 (Tokyo)","|Nothing To Do|","",""
 "`LWG4031 <https://wg21.link/LWG4031>`__","``bad_expected_access<void>`` member functions should be ``noexcept``","2024-03 (Tokyo)","|Complete|","16.0",""
 "`LWG4035 <https://wg21.link/LWG4035>`__","``single_view`` should provide ``empty``","2024-03 (Tokyo)","|Complete|","19.0",""

libcxx/include/__expected/expected.h

@@ -1493,8 +1493,6 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
     return *this;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(expected&&) = delete;
-
   _LIBCPP_HIDE_FROM_ABI constexpr expected&
   operator=(expected&& __rhs) noexcept(is_nothrow_move_assignable_v<_Err> && is_nothrow_move_constructible_v<_Err>)
     requires(is_move_assignable_v<_Err> && is_move_constructible_v<_Err>)