Fix visible forall in ppr_ty (#18522)
Before this patch, this type:
T :: forall k -> (k ~ k) => forall j -> k -> j -> Type
was printed incorrectly as:
T :: forall k j -> (k ~ k) => k -> j -> Type
Whereas #18458 would make this workaround obsolete, there's no consensus as to what the best way to do #18458 would be; so I figured I'd fix the immediate issue in ppr_ty
.