Lazy goto model: allow access to mutable goto_functiont

This was previously forbidden because with symex-driven loading it's
always a mistake to edit a function after symex has begun executing it,
but with any load strategy which takes place entirely before symex it's
ok if you're careful.

Author: smowton

src/goto-programs/lazy_goto_model.h

@@ -243,6 +243,16 @@ class lazy_goto_modelt : public abstract_goto_modelt
     return goto_functions.at(id);
   }
 
+  /// Get a goto_function_body, for the the purpose of updating it.
+  /// With symex-driven loading it's a mistake to update a function after symex
+  /// has begun executing it. However with any load strategy which takes place
+  /// entirely before symex it is not a problem.
+  goto_functionst::goto_functiont &
+  get_goto_function_writable(const irep_idt &id)
+  {
+    return goto_functions.at(id);
+  }
+
   /// Check that the goto model is well-formed
   ///
   /// The validation mode indicates whether well-formedness check failures are
@@ -263,7 +273,7 @@ class lazy_goto_modelt : public abstract_goto_modelt
   symbol_tablet &symbol_table;
 
 private:
-  const lazy_goto_functions_mapt goto_functions;
+  lazy_goto_functions_mapt goto_functions;
   language_filest language_files;
 
   // Function/module processing functions