make the zero-bit-ness of pointers lazy

this case works now:

```zig
const Foo = struct {
    field: @typeOf(func).ReturnType,
};
fn func(self: *Foo) void {}
```
64e9b0ee46Andrew Kelley committed on 8/26/2019, 12:27:56 AMparent8f41da2
2 files changedLine totals unavailable