Summary:
For the sake of consistency of the dependent core, there is a restriction on
where a coercion variable can appear in ForAllCo: the coercion variable can
appear nowhere except in coherence coercions.

Currently this restriction is missing in Core. The goal of this patch is to add
the missing restriction.

After discussion, we decide: coercion variables can appear nowhere except in
`GRefl` and `Refl`. Relaxing the restriction to include `Refl` should not break
consistency, we premuse.

Test Plan: ./validate

Reviewers: goldfire, simonpj, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, carter

GHC Trac Issues: #15757

Differential Revision: https://phabricator.haskell.org/D5231