rustc: Improve errors on private static methods

This gives a better NOTE error message when a privacy error is encountered with
a static method. Previously no note was emitted (due to lack of support), but
now a note is emitted indicating that the struct/enum itself is private.

Closes #13641

src/test/compile-fail/issue-13641.rs

@@ -0,0 +1,25 @@
+// 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.
+
+mod a {
+    struct Foo;
+    impl Foo { pub fn new() {} }
+    enum Bar {}
+    impl Bar { pub fn new() {} }
+}
+
+fn main() {
+    a::Foo::new();
+    //~^ ERROR: static method `new` is inaccessible
+    //~^^ NOTE: struct `Foo` is private
+    a::Bar::new();
+    //~^ ERROR: static method `new` is inaccessible
+    //~^^ NOTE: enum `Bar` is private
+}