Fix goto program hash function

The hash function for goto program targets previously dereferenced the
target and returned the instruction's location number. However, in some
circumstances the location number of an instruction might be modified.
If some unordered_map/set is using an iterator to a modified instruction
as a key, then the collection invariants will be broken, leading to
undefined behaviour.

This patch switches to using instruction addresses as hash values,
allowing any field in the instruction to be modified without affecting
the hash result.

Author: reuk

src/goto-programs/goto_program_template.h

@@ -807,7 +807,10 @@ struct const_target_hash_templatet
 {
   std::size_t operator()(
     const typename goto_program_templatet<codeT, guardT>::const_targett t) const
-  { return t->location_number; }
+  {
+    using hash_typet = decltype(&(*t));
+    return std::hash<hash_typet>{}(&(*t));
+  }
 };
 
 /// Functor to check whether iterators from different collections point at the