Summary:
Phabricator/Harbormaster measured the following allocation numbers for
haddock.compiler:

Expected:     33562468736
June 2nd [1]: 36740649320
Upper bound:  36918715610
June 3rd [2]: 36956620504

So although this test didn't start failing until June 2nd/3rd, the
biggest increase in allocation must have occured sometime before that.

[1] 2f0011ac
[2] 942cfa4e

Test Plan: validate

Reviewers: austin

Subscribers: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D948