Regression: repr(C) on typedef

http://is.gd/uaDffj

```
#[repr(C)]
type Foo = extern fn();

fn main(){}
```

compiles on stable, not on beta.

This probably shouldn't compile in the first place since typedefs are aliases, but it's better to phase it out and replace it with a warning (later an error) instead of breaking it completely.

cc @brson 
