Fix :info pretty-printing of UNPACKed fields

This patch:

* Ensures that we do not pretty-print a field like `foo :: {-# UNPACK #-} !Int`
  as `foo :: ! {-# UNPACK -#} Int` (as we were doing before) when running the
  `:info` command.
* Prevents coercions that arise from `UNPACK`ed fields (e.g., such as when one
  unpacks a newtype) from being printed in `:info` output unless `-dppr-debug`
  is enabled.

Fixes #25651.