[mlir:OpFormatGen] Add Support for `$_ctxt` in the transformer.

This is useful for "build tuple" type ops. In my case, in npcomp, I have
an op:

```
// Result type is `!torch.tuple<!torch.tensor, !torch.tensor>`.
torch.prim.TupleConstruct %0, %1 : !torch.tensor, !torch.tensor
```

and the context is required for the `Torch::TupleType::get` call (for
the case of an empty tuple).

The handling of these FmtContext's in the code is pretty ad-hoc -- I didn't
attempt to rationalize it and just made a targeted fix. As someone
unfamiliar with the code I had a hard time seeing how to more broadly fix
the situation.

Differential Revision: https://reviews.llvm.org/D104274