Return error for non-constructor functions returning external objects

[Martin Sjölund]

Code like the following is dangerous, because the Id destructor will be called on function exit (since it is a local variable...). Make the front-end return an error here instead.

import Id = ModelicaServices.modcount.HeapString;

function shapeDescrTo3D
  input State state;
  input String descr;
  input Real length, width, height;
  input Real[3] at;
  input Real extra;
  output Id id;