Currently ap_0_fast doesn't maintain the invariant for PAP fun fields
which says if the closure can be tagged, it should be. This is checked
by `Sanity.c:checkPAP` and correctly implemented by `genautoapply`.

This causes sanity check failures when we have a profiling code like

    f = {-# SCC scc #-} g

where g is a PAP or a FUN, and `scc` is different than the current cost
centre.

Test Plan: Slow validate (not done yet)

Reviewers: simonmar, bgamari, erikd

Reviewed By: simonmar

Subscribers: rwbarton, carter

GHC Trac Issues: #15508

Differential Revision: https://phabricator.haskell.org/D5051