deprecate codet::make_block

This method isn't type safe. It has no users in the code base.

Author: Daniel Kroening

src/util/std_code.h

@@ -62,6 +62,8 @@ class codet:public exprt
   const codet &first_statement() const;
   codet &last_statement();
   const codet &last_statement() const;
+
+  DEPRECATED("use code_blockt(...) instead")
   class code_blockt &make_block();
 
   /// Check that the code statement is well-formed (shallow checks only, i.e.,