Fix the unused struct field lint for struct variants

Fixes #14837.

src/test/run-pass/issue-14837.rs

@@ -0,0 +1,20 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(struct_variant)]
+#![deny(warnings)]
+
+pub enum Foo {
+    Bar {
+        pub baz: int
+    }
+}
+
+fn main() { }