Don't let a type parameter named "Self" unchanged past HIR lowering.

src/test/compile-fail/issue-36638.rs

@@ -0,0 +1,19 @@
+// Copyright 2016 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.
+
+// compile-flags: -Z continue-parse-after-error
+
+struct Foo<Self>(Self);
+//~^ ERROR expected identifier, found keyword `Self`
+
+trait Bar<Self> {}
+//~^ ERROR expected identifier, found keyword `Self`
+
+fn main() {}