Parse fully-qualified associated types in generics without whitespace

This breaks code that looks like this:

    let x = foo as bar << 13;

Change such code to look like this:

    let x = (foo as bar) << 13;

Closes #17362.

[breaking-change]

src/test/bench/shootout-reverse-complement.rs

@@ -62,7 +62,7 @@ impl Tables {
         }
         let mut table16 = [0;1 << 16];
         for (i, v) in table16.iter_mut().enumerate() {
-            *v = table8[i & 255] as u16 << 8 |
+            *v = (table8[i & 255] as u16) << 8 |
                  table8[i >> 8]  as u16;
         }
         Tables { table8: table8, table16: table16 }

src/test/run-pass/parse-assoc-type-lt.rs

@@ -0,0 +1,18 @@
+// 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.
+
+#![feature(associated_types)]
+
+trait Foo {
+    type T;
+    fn foo() -> Box<<Self as Foo>::T>;
+}
+
+fn main() {}