Fix #15637 by using VTA more in GND

Summary:
The code that GND was generating before could crumple over
if it derived an instance for a class with an ambiguous type variable
in the class head, such as the example in #15637. The solution is
straightforward: simply instantiate all variables bound by the class
head explicitly using visible type application, which will nip any
ambiguity in the bud.

Test Plan: make test TEST=T15637

Reviewers: bgamari, simonpj, goldfire

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, carter

GHC Trac Issues: #15637

Differential Revision: https://phabricator.haskell.org/D5148