Fix ICE on inner enum in missing docs lint.

This also simplifies the lint by not storing variant depth or the struct
def stack, because we no longer need them.

src/test/compile-fail/issue-40350.rs

@@ -0,0 +1,24 @@
+// Copyright 2017 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(rustc_attrs)]
+#![allow(warnings)]
+
+enum E {
+    A = {
+        enum F { B }
+        0
+    }
+}
+
+#[rustc_error]
+fn main() {}
+//~^ ERROR compilation successful
+