rustc_resolve: Do not allow mods to shadow types

This commit modifies resolve to prevent conflicts with typedef names in the same
method that conflits are prevented with enum names. This is a breaking change
due to the differing semantics in resolve, and any errors generated on behalf of
this change require that a conflicting typedef, module, or structure to be
renamed so they do not conflict.

[breaking-change]
Closes #6936

src/test/compile-fail/issue-6936.rs

@@ -0,0 +1,44 @@
+// 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.
+
+struct T;
+
+mod t1 {
+    type Foo = ::T;
+    mod Foo {} //~ ERROR: duplicate definition of type or module `Foo`
+}
+
+mod t2 {
+    type Foo = ::T;
+    struct Foo; //~ ERROR: duplicate definition of type or module `Foo`
+}
+
+mod t3 {
+    type Foo = ::T;
+    enum Foo {} //~ ERROR: duplicate definition of type or module `Foo`
+}
+
+mod t4 {
+    type Foo = ::T;
+    fn Foo() {} // ok
+}
+
+mod t5 {
+    type Bar<T> = T;
+    mod Bar {} //~ ERROR: duplicate definition of type or module `Bar`
+}
+
+mod t6 {
+    type Foo = ::T;
+    impl Foo {} // ok
+}
+
+
+fn main() {}