Should `this` argument types be banned in interfaces?

### HHVM Version

```
$ hhvm --version
HipHop VM 3.19.1 (rel)
Compiler: tags/HHVM-3.19.1-0-g6129094b1b61e1ae32245a583db0283e645384c1
Repo schema: 062f6c54b49727e83a10a43cdbb861741076e51d
```

### Standalone code, or other way to reproduce the problem

```hack

Use the standalone Hack interface example as the reproducer and begin by locating the typechecker entry point for `this` types in contravariant positions. Done means the invalid interface declaration produces a targeted type error and the behavior is covered by a regression test.

Written by the indexing model from the issue text.