add missing output for ui test

ExpHP · ExpHP · commit fbc57a7c7a9c · 2018-05-03T00:25:14.000-04:00
diff --git a/src/test/ui/issue-50403.stderr b/src/test/ui/issue-50403.stderr
@@ -0,0 +1,8 @@
+error: concat_idents! takes 1 or more arguments.
+  --> $DIR/issue-50403.rs:14:13
+   |
+LL |     let x = concat_idents!(); //~ ERROR concat_idents! takes 1 or more arguments
+   |             ^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
