[patch][pr17535] Reject alias attributes that point to undefined names.

[Rafael Espíndola]

> Great, I think diagnosing at CodeGen is the better choice here.
>
> Rafael, in the new patch could you swap around one of the tests to prove
> that checking isn't dependent on parse order?
>
> -void alias1() {}
>  void alias2 [[gnu::alias("_Z6alias1v")]] ();
> +void alias1() {}

Since that was already passing, I just added to the test in r193099.

Cheers,
Rafael