This isn't strictly necessary for deterministic ABIs.
The results of eltsHpt are consumed in two ways:
1) they determine the order of linking
2) if you track the data flow all the family instances get put in
   FamInstEnvs, so the nondeterministic order is forgotten.
3) same for VectInfo stuff
4) same for Annotations

The problem is that I haven't found a nice way to do 2. in
a local way and 1. is nice to have if we went for deterministic
object files. Besides these maps are keyed on ModuleNames so they
should be small relative to other things and the overhead should
be negligible.

As a bonus we also get more specific names.

Test Plan: ./validate

Reviewers: bgamari, austin, hvr, ezyang, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2300

GHC Trac Issues: #4012