Change as_string to id2string

Author: angelhof

src/ansi-c/c_typecheck_type.cpp

@@ -1241,7 +1241,7 @@ void c_typecheck_baset::typecheck_c_enum_type(typet &type)
   {
     // Note: This is inefficient as it first turns integers to strings
     // and then turns them back to bvrep
-    auto value = string2integer(as_string(member.get_value()));
+    auto value = string2integer(id2string(member.get_value()));
     member.set_value(integer2bvrep(value, width));
   }