Warn when creating a module and a struct that both have the same name.

Currently it is possible to do the following:

 - define a module named `Foo` and then a unit or tuple struct also named `Foo`
 - define any struct named `Foo` and then a module named `Foo`

This commit introduces a warning for both of these cases.

src/test/compile-fail/issue-21546.rs

@@ -0,0 +1,65 @@
+// 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.
+
+// Also works as a test for #14564
+
+#[allow(non_snake_case)]
+mod Foo { }
+//~^ NOTE first definition of type or module `Foo`
+
+#[allow(dead_code)]
+struct Foo;
+//~^ WARNING duplicate definition of type or module `Foo`
+
+
+#[allow(non_snake_case)]
+mod Bar { }
+//~^ NOTE first definition of type or module `Bar`
+
+#[allow(dead_code)]
+struct Bar(i32);
+//~^ WARNING duplicate definition of type or module `Bar`
+
+
+#[allow(dead_code)]
+struct Baz(i32);
+//~^ NOTE first definition of type or module
+
+#[allow(non_snake_case)]
+mod Baz { }
+//~^ WARNING duplicate definition of type or module `Baz`
+
+
+#[allow(dead_code)]
+struct Qux { x: bool }
+//~^ NOTE first definition of type or module
+
+#[allow(non_snake_case)]
+mod Qux { }
+//~^ WARNING duplicate definition of type or module `Qux`
+
+
+#[allow(dead_code)]
+struct Quux;
+//~^ NOTE first definition of type or module
+
+#[allow(non_snake_case)]
+mod Quux { }
+//~^ WARNING duplicate definition of type or module `Quux`
+
+
+#[allow(dead_code)]
+enum Corge { A, B }
+
+#[allow(non_snake_case)]
+mod Corge { }
+//~^ ERROR duplicate definition of type or module `Corge`
+
+fn main() { }