Refactor move(init_type&) to handle defects in C++20 versions of libstdc++ for gcc versions <=12

include/boost/unordered/unordered_flat_map.hpp

@@ -55,7 +55,7 @@ namespace boost {
 
         static moved_type move(init_type& x)
         {
-          return {std::get<0>(std::move(x)), std::get<1>(std::move(x))};
+          return {std::move(x.first), std::move(x.second)};
         }
 
         static moved_type move(element_type& x)

include/boost/unordered/unordered_node_map.hpp

@@ -79,7 +79,7 @@ namespace boost {
         static element_type&& move(element_type& x) { return std::move(x); }
         static moved_type move(init_type& x)
         {
-          return {std::get<0>(std::move(x)), std::get<1>(std::move(x))};
+          return {std::move(x.first), std::move(x.second)};
         }
 
         static moved_type move(value_type& x)