Collect CCs in CorePrep, including CCs in unfoldings

This patch includes two changes:

1. Move cost centre collection from `SCCfinal` to `CorePrep`, to be able
   to collect cost centres in unfoldings. `CorePrep` drops unfoldings, so
   that's the latest stage in the compilation pipeline for this.

   After this change `SCCfinal` no longer collects all cost centres, but
   it still generates & collects CAF cost centres + updates cost centre
   stacks of `StgRhsClosure` and `StgRhsCon`s.

   This fixes #5889.

2. Initialize cost centre stack fields of `StgRhs` in `coreToStg`. With
   this we no longer need to update cost centre stack fields in
   `SCCfinal`, so that module is removed.

   Cost centre initialization explained in Note [Cost-centre
   initialization plan].

   Because with -fcaf-all we need to attach a new cost-centre to each
   CAF, `coreTopBindToStg` now returns `CollectedCCs`.

Test Plan: validate

Reviewers: simonpj, bgamari, simonmar

Reviewed By: simonpj, bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #5889

Differential Revision: https://phabricator.haskell.org/D4325