rwbarton authored Mar 03, 2017 and Ben Gamari committed Mar 03, 2017

This change sped up the total validate --build-only time by 0.8%
on my test system; hopefully a representative result.

I didn't bother making the other constructors lazy because for
IfaceData and IfaceClass we need to pull on some of the fields
in loadDecl, and all the others seem much more rare than IfaceId.

Test Plan: validate, perf

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3269