The optimisation in write_remoting_arg for sizing, marshalling,
unmarshalling and freeing ref pointers to base types directly was
broken in d458a599 and caused
unnecessary calls to pointer marshalling functions.

The same was true for a similar optimisation for simple structures
where their size can be pre-calculated, freeing omitted and the
NdrSimpleStructMarshall/Unmarshall functions called directly instead
of calling NdrPointerMarshall/Unmarshall first.

Fix this by looking at the type of the referrent instead of the type
of the pointer, making sure to not classify user types as simple
structures.

Additionally remove some dead code that would never be executed
because the code is in an RPC_FC_RP case, inside a !is_ptr if block
(and RPC_FC_RP is a pointer type).

widl: Output the correct buffer size required for parameters whose sizes are known at type-generation time.

Replace code to calculate the size of RPC_FC_STRUCT types with
fields_memsize since the memory size of these types will always be
equal to the buffer size.

Remove dead code in get_required_buffer_size.

widl: Do not make the stub vtbl const for delegating interfaces since it needs to be patched up at run-time.