ICE on Unicode escape in byte string literal

``` rust
fn main() {
    b"\u{a66e}";
}
```

```
thread 'rustc' panicked at 'lexer accepted invalid byte literal \u{a66e} step 3', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/parse/mod.rs:592
```
