syntax::fold::make_fold leaks

```
use rustc;
import rustc::syntax::fold;

fn main()
{
    auto afp = fold::default_ast_fold();
    auto af = fold::make_fold(*afp);
}
```

rt: fatal, 'leaked memory in rust main loop (23 objects)' failed, ../src/rt/memory_region.cpp:156 23 objects
