Cleanup: simplify subclassing CPPType

`CPPType` can wrap any C++ type so that code can work
with the wrapped type in a generic way. The goal of subclassing
`CPPType` is to provide additional methods for some types.
For example, the `CPPType` for `Array<int>` could have a `.element_type()`
method that returns the `CPPType` for `int`.

source/blender/functions/intern/multi_function_network_optimization.cc

@@ -263,7 +263,7 @@ static Array<MFOutputSocket *> add_constant_folded_sockets(const MultiFunction &
         const CPPType &cpp_type = data_type.single_type();
         GMutableSpan array = params.computed_array(param_index);
         void *buffer = array.data();
-        scope.add(buffer, array.type().members().destruct, AT);
+        scope.add(buffer, array.type().destruct_fn(), AT);
 
         constant_fn = &scope.construct<CustomMF_GenericConstant>(AT, cpp_type, buffer);
         break;