Summary:
Currently, `CoreToStg` annotates `StgRhsClosure`s with their set of non-global
free variables.  This free variable information is only needed in the final
code generation step (i.e. `StgCmm.codeGen`), which leads to transformations
such as `StgCse` and `StgUnarise` having to maintain this information.

This is tiresome and unnecessary, so this patch introduces a trees-to-grow-like
approach that only introduces the free variable set into the syntax tree in the
code gen pass, along with a free variable analysis on STG terms to generate
that information.

Fixes #15754.

Reviewers: simonpj, osa1, bgamari, simonmar

Reviewed By: osa1

Subscribers: rwbarton, carter

GHC Trac Issues: #15754

Differential Revision: https://phabricator.haskell.org/D5324