forceValue: make pos mandatory

- Make passing the position to `forceValue` mandatory,
  this way we remember people that the position is
  important for better error messages
- Add pos to all `forceValue` calls

src/nix-instantiate/nix-instantiate.cc

@@ -40,7 +40,7 @@ void processExpr(EvalState & state, const Strings & attrPaths,
 
     for (auto & i : attrPaths) {
         Value & v(*findAlongAttrPath(state, i, autoArgs, vRoot).first);
-        state.forceValue(v);
+        state.forceValue(v, v.determinePos(noPos));
 
         PathSet context;
         if (evalOnly) {