goto-symex: Print (at debug level) the current assertion being converted

For other step types this had already been done in "Print (at debug level) the
current SSA step being converted."

Author: tautschnig

src/goto-symex/symex_target_equation.cpp

@@ -623,6 +623,14 @@ void symex_target_equationt::convert_assertions(
   {
     if(step.is_assert())
     {
+      prop_conv.conditional_output(
+        prop_conv.debug(),
+        [&step](messaget::mstreamt &mstream) {
+          std::ostringstream oss;
+          step.output(oss);
+          mstream << oss.str() << messaget::eom;
+        });
+
       implies_exprt implication(
         assumption,
         step.cond_expr);