Trac #14845 brought to light a corner case where a data
constructor could not be promoted (even with `-XTypeInType`) due to
an unpromotable constraint in its context. However, the error message
was less than helpful, so this patch adds an additional check to
`tcTyVar` catch unpromotable data constructors like these //before//
they're promoted, and to give a sensible error message in such cases.

Test Plan: make test TEST="T13895 T14845"

Reviewers: simonpj, goldfire, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #13895, #14845

Differential Revision: https://phabricator.haskell.org/D4728