deprecate exprt::make_typecast()

This is fundamentally not type-safe; the type of the expression will change.
The compiler is unable to assert memory safety.

Author: Daniel Kroening

src/util/expr.h

@@ -212,6 +212,7 @@ class exprt:public irept
     op.push_back(std::move(e3));
   }
 
+  DEPRECATED("use typecast_exprt() instead")
   void make_typecast(const typet &_type);
 
   void make_bool(bool value);