Fix issue where trait-associated consts could cause ICEs in match patterns.

This allows some lookup of trait-associated consts during type-checking,
which may be helpful for future fixes as well.

src/test/run-pass/associated-const-range-match-patterns.rs

@@ -0,0 +1,35 @@
+// 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.
+
+#![feature(associated_consts)]
+
+struct Foo;
+
+trait HasNum {
+    const NUM: isize;
+}
+impl HasNum for Foo {
+    const NUM: isize = 1;
+}
+
+fn main() {
+    assert!(match 2 {
+        Foo::NUM ... 3 => true,
+        _ => false,
+    });
+    assert!(match 0 {
+        -1 ... <Foo as HasNum>::NUM => true,
+        _ => false,
+    });
+    assert!(match 1 {
+        <Foo as HasNum>::NUM ... <Foo>::NUM => true,
+        _ => false,
+    });
+}