Common type of ubyte and const ubyte is int

[Steven Schveighoffer]

```d
ubyte a;
const(ubyte) b;

auto x = true ? a : b;

pragma(msg, typeof(x)); // int
```

Why? This isn't a VRP problem, as both are `ubyte`.

Related issue

https://issues.dlang.org/show_bug.cgi?id=19817

-Steve