libstdc++: Make std::move_only_function never valueless in std::variant

This is not suitable to backport, as it affects the ABI of std::variant
and so isn't appropriate for a release branch.

libstdc++-v3/ChangeLog:

	* include/bits/move_only_function.h (_Never_valueless_alt):
	Define partial specialization for std::move_only_function.

libstdc++-v3/include/bits/move_only_function.h

@@ -183,6 +183,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     constexpr bool __is_move_only_function_v<move_only_function<_Tp>> = true;
   /// @endcond
 
+  namespace __detail::__variant
+  {
+    template<typename> struct _Never_valueless_alt; // see <variant>
+
+    // Provide the strong exception-safety guarantee when emplacing a
+    // move_only_function into a variant.
+    template<typename... _Signature>
+      struct _Never_valueless_alt<std::move_only_function<_Signature...>>
+      : true_type
+      { };
+  }  // namespace __detail::__variant
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std