Replace guardt in dereference_failure by exprt

This was immediately converted to an exprt anyway, so doesn't need to be
a guardt specifically.

Author: romainbrenguier

src/pointer-analysis/goto_program_dereference.cpp

@@ -68,10 +68,8 @@ bool goto_program_dereferencet::is_valid_object(
 void goto_program_dereferencet::dereference_failure(
   const std::string &property,
   const std::string &msg,
-  const guardt &guard)
+  exprt guard_expr)
 {
-  exprt guard_expr=guard.as_expr();
-
   if(assertions.insert(guard_expr).second)
   {
     guard_expr = boolean_negate(guard_expr);

src/pointer-analysis/goto_program_dereference.h

@@ -19,8 +19,6 @@ Author: Daniel Kroening, [email protected]
 #include "value_sets.h"
 #include "value_set_dereference.h"
 
-class guardt;
-
 /// Wrapper for functions removing dereferences in expressions contained in
 /// a goto program.
 class goto_program_dereferencet:protected dereference_callbackt
@@ -77,7 +75,7 @@ class goto_program_dereferencet:protected dereference_callbackt
   virtual void dereference_failure(
     const std::string &property,
     const std::string &msg,
-    const guardt &guard);
+    exprt guard_expr);
 
   void
   get_value_set(const exprt &expr, value_setst::valuest &dest) const override;