Warn on usage of goto-instrument --constant-propagator

According to the doxygen in `analyses/constant_propagator.h`, this is "A
simple, unsound constant propagator." It is worth warning users so that
it is not accidentally applied to use cases where soundness is
important.

Author: thomasspriggs

src/goto-instrument/goto_instrument_parse_options.cpp

@@ -1288,6 +1288,10 @@ void goto_instrument_parse_optionst::instrument_goto_program()
     do_remove_returns();
 
     log.status() << "Propagating Constants" << messaget::eom;
+    log.warning() <<
+      "**** WARNING: Constant propagation as performed by goto-instrument is "
+      "not expected to be sound. Do not use --constant-propagator if soundness "
+      "is important for your use case." << messaget::eom;
 
     constant_propagator_ait constant_propagator_ai(goto_model);
     remove_skip(goto_model);