NF treats polymorphic as a reserved type in Modelica code

[Per Östlund]

This model:

model M
  class polymorphic
    Real x;
  end polymorphic;

  polymorphic x;
end M;

flattens to this:

class M
  polymorphic<> x;
end M;

polymorphic is an implementation detail for generic types that shouldn't leak into normal Modelica code.