name mangling seems to miss generic class arguments in some cases

```carbon
class C {}
class C2 {}

class D(T:! type) {}

interface I {
  fn F();
}

impl D(C) as I {
  fn F() {}
}

impl D(C2) as I {
  fn F() {}
}
```

results in an error due to a mangling collision. Because the two `impl`s aren't generic, we appear to give them the same mangling, even though we need to include the specific ID for `D` in order to distinguish them.