Prevent solveFlatWanteds from losing insolubles when using typechecker plugins

Summary: I've added a Note explaining the problem.

Test Plan:
validate; we don't have a very good story for testing plugins yet,
but I've verified that this does at least fix the bug in my plugin.

Reviewers: simonpj, austin

Reviewed By: austin

Subscribers: carter, thomie, gridaphobe, yav

Differential Revision: https://phabricator.haskell.org/D552