Add a -fprint-axiom-incomps option (#15546)
Supply branch incomps when building an IfaceClosedSynFamilyTyCon
pprTyThing now has access to incomps. This also causes them to be
written out to .hi files, but that doesn't pose an issue other than a
more faithful bijection between
The machinery for displaying axiom incomps was already present but not in use. Since this is now a thing that pops up in ghci's :info the format was modified to look like a haskell comment.
Documentation and a test for the new feature included.
Test Plan: T15546
Reviewers: simonpj, bgamari, goldfire
Reviewed By: simonpj
Subscribers: rwbarton, carter
GHC Trac Issues: #15546 (closed)
Differential Revision: https://phabricator.haskell.org/D5097