Suggest how to fix illegally nested foralls in GADT constructor type signatures

Summary:
Although the code from #12087 isn't accepted by GHC, we can at least
do a better job of letting users know what the problem is, and how to fix it.

Test Plan: make test TEST=T12087

Reviewers: goldfire, austin, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, thomie

GHC Trac Issues: #12087

Differential Revision: https://phabricator.haskell.org/D3851