Use generic exprt to avoid deprecated default constructor of symbol_exprt

The implementation here is highly incomplete, in particular those symbols are
never set. If at some future date the implementation is completed then the type
of those members might be changed again.

Author: tautschnig

src/goto-instrument/concurrency.cpp

@@ -58,14 +58,14 @@ class concurrency_instrumentationt
   {
   public:
     typet type;
-    symbol_exprt array_symbol, w_index_symbol;
+    exprt array_symbol, w_index_symbol;
   };
 
   class thread_local_vart
   {
   public:
     typet type;
-    symbol_exprt array_symbol;
+    exprt array_symbol;
   };
 
   typedef std::map<irep_idt, shared_vart> shared_varst;