Summary:
`okConIdOcc`, which validates that a type or constructor name is valid
for splicing using Template Haskell, has a special case for tuples, but
neglects to look for unboxed tuples, causing some sensible Template Haskell
code involving unboxed tuples to be rejected.

Fixes #12407.

Test Plan: make test TEST=T12407

Reviewers: austin, bgamari, hvr, goldfire

Reviewed By: goldfire

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2410

GHC Trac Issues: #12407