r178122 - Fix testcase to add expected note.

[Tim Northover]

Hi,

> +struct S;  // expected-note {{forward declaration of 'struct S'}}
> should expect 2 notes.

Looks right. I'm assuming Bill is asleep right now, so I've committed
a change implementing that (r178143). It can be reverted if it's not
what he intended, and meanwhile we get passing tests during our day.

Cheers.

Tim.