add warning for calling pure nothrow functions and ignoring the result

Author: WalterBright

src/expression.c

@@ -7548,8 +7548,8 @@ int CallExp::checkSideEffect(int flag)
            )
         {
             result = 0;
-            //if (flag == 0)
-                //warning("pure nothrow function %s has no effect", e1->toChars());
+            if (flag == 0)
+                warning("pure nothrow function %s has no effect", e1->toChars());
         }
         else
             result = 1;