Summary:
Previously, one could experience error cascades with deriving clauses
when one class in a set of many failed to derive, causing the other derived
classes to be skipped entirely and resulting in other errors down the line.
The solution is to process each class in a data type's set of deriving clauses
individually, and engineer it so that failure to derive an individual class
within that set doesn't cancel out the others.

Test Plan: make test TEST="T10684 T12801"

Reviewers: austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie

GHC Trac Issues: #10684, #12801

Differential Revision: https://phabricator.haskell.org/D3771