parameter of type &const namespace.T drops const qualifier

```zig
// broken: drops const qualifier
fn createPrefixOp(op: &const ast.NodePrefixOp.PrefixOp) {}

// works
const WorkaroundPrefixOp = ast.NodePrefixOp.PrefixOp;
fn createPrefixOp(op: &const WorkaroundPrefixOp) {}
```