lint: handle private traits better

Due to a long-standing conservative approach to trait exports, all traits are
considered exported. However, the missing_docs lint uses the export map to
determine if something is public and ought to have documentation. This commit
modifies the lint to check if traits are private before emitting the warning.

Closes #11592

src/test/run-pass/issue-11592.rs

@@ -0,0 +1,20 @@
+// Copyright 2015 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.
+
+//! Ensure the private trait Bar isn't complained about.
+
+#![deny(missing_docs)]
+
+mod foo {
+    trait Bar { fn bar(&self) { } }
+    impl Bar for i8 { fn bar(&self) { } }
+}
+
+fn main() { }