When outputting list of available instances, sort it.

Summary:
The intent of this commit is to make test suite cases more stable, so that
it doesn't matter what order we load interface files in, the test output
doesn't change.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, austin

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D484