Strict TTG extension fields

Currently, the TTG extension fields are not strict:

data HsType pass
  = ...
  | HsTyVar  (XTyVar pass) PromotionFlag (LIdP pass)
  | HsAppTy  (XAppTy pass) (LHsType pass) (LHsType pass)
  | ...

But they very well could be:

data HsType pass
  = ...
  | HsTyVar  !(XTyVar pass) PromotionFlag (LIdP pass)
  | HsAppTy  !(XAppTy pass) (LHsType pass) (LHsType pass)
  | ...

This would allow one to "exclude" certain constructors by using Void:

type instance XTyVar GhcTc = Void

And then the pattern match exhaustiveness checker would see that HsTyVar is an impossible case, without an explicit call to absurd. We already do that for sum extension fields, but for some reason not for all of them.

It's easy to recover laziness by using a wrapper, at the cost of a pointer indirection:

data Box a = Box a

I propose to make all extension fields throughout the compiler strict. Thoughts?