Add hanlded for constant strings

The data that was missing from the trace was for the storing of the
class identifier. This was a constant string assignment step. At some
point the handling for filling in the value structure got lost. This
reintrodues that change.

Author: thk123

src/util/json_expr.cpp

@@ -347,6 +347,11 @@ json_objectt json(
       to_integer(to_constant_expr(expr), b);
       result["data"]=json_stringt(integer2string(b));
     }
+    else if(type.id()==ID_string)
+    {
+      result["name"]=json_stringt("string");
+      result["data"]=json_stringt(expr.get_string(ID_value));
+    }
     else
     {
       result["name"]=json_stringt("unknown");