Allow attributes on match arms

Implements RFC 8
auto merge of #12812 : sfackler/rust/attr-arm, r=alexcrichton

This is really only useful for #[cfg()]. For example:

```rust
enum Foo {
    Bar,
    Baz,
    #[cfg(blob)]
    Blob
}

fn match_foos(f: &Foo) {
    match *f {
        Bar => {}
        Baz => {}
        #[cfg(blob)]
        Blob => {}
    }
}
```

This is a kind of weird place to allow attributes, so it should probably
be discussed before merging.