Manually-written instances and standalone-derived instances
have the benefit of having the `checkValidInstHead` function run over
them, which catches manual instances of built-in types like `(~)` and
`Coercible`. However, instances generated from `deriving` clauses
weren't being passed through `checkValidInstHead`, leading to
confusing results as in #14916.

`checkValidInstHead` also has additional validity checks for language
extensions like `FlexibleInstances` and `MultiParamTypeClasses`. Up
until now, GHC has never required these language extensions for
`deriving` clause, so to avoid unnecessary breakage, I opted to
suppress these language extension checks for `deriving` clauses, just
like we currently suppress them for `SPECIALIZE instance` pragmas.

Test Plan: make test TEST=T14916

Reviewers: goldfire, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14916

Differential Revision: https://phabricator.haskell.org/D4501