goto_statet cannot have default assignment operator

It uses classes that don't have a default assignment operator.  This change
is required for compilation with clang 8.

Author: Daniel Kroening

src/goto-symex/goto_state.h

@@ -63,7 +63,7 @@ class goto_statet
 
   /// Constructors
   goto_statet() = default;
-  goto_statet &operator=(const goto_statet &other) = default;
+  goto_statet &operator=(const goto_statet &other) = delete;
   goto_statet &operator=(goto_statet &&other) = default;
   goto_statet(const goto_statet &other) = default;
   goto_statet(goto_statet &&other) = default;