This includes two bug fixes in profiling version of stg_ap_0_fast:

- PAPs allocated by stg_ap_0_fast are now correctly tagged. This
  invariant is checked in Sanity.c:checkPAP.

  (This was originally implemented in 2693eb11, later reverted with
  ab55b4dd because it revealed the bug below, but it wasn't clear at
  the time whether the bug was the one below or something in the commit)

- The local variable `untaggedfun` is now marked as a pointer so it
  survives GC.

With this we finally fix all known bugs caught in #15508. `concprog001`
now works reliably with prof+threaded and prof runtimes (with and
without -debug).

(cherry picked from commit 908b4b86)