Fix retainer profiling after SRT overhaul

Authored by simonmar on May 19 2018, 12:43 AM.

Description

Fix retainer profiling after SRT overhaul

Recent changes to SRTs (D4632, D4637) also required changes to
RetainerProfile.c. This should hopefully get things working again.

Test Plan: validate with profiling turned on

Reviewers: bgamari, osa1, tdammers, erikd

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4707