[ty] Fix: override all fail_* methods for variant types in comparator_ty
[ty] Fix: override all fail_* methods for variant types in comparator_ty

Summary:
An awkward caveat of using `iter2` to implement a comparator for `Ty.t` is that
we need to override all `fail_*` methods to avoid `VisitorsRuntime.StructuralMismatch`
exceptions.

This diff fixes a few of those cases that had regressed lately.

Reviewed By: samwgoldman

Differential Revision: D21577992

fbshipit-source-id: 3c2e25cf26ecd3918cf33184eab57fcd7d2d2d15

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow