Add function id to value-set.

Value sets already know their own location id; this adds the function
id too so that context-sensitive variants of VSA can find that without
needing to track extra context themselves.

Author: smowton

src/pointer-analysis/value_set.h

@@ -32,6 +32,7 @@ class value_sett
     const namespacet &);
 
   unsigned location_number;
+  irep_idt function;
   static object_numberingt object_numbering;
 
   typedef irep_idt idt;

src/pointer-analysis/value_set_domain.h

@@ -39,6 +39,7 @@ class value_set_domaint:public domain_baset
   {
     value_set.clear();
     value_set.location_number=l->location_number;
+    value_set.function=l->function;
   }
 
   virtual void transform(