Simon Marlow authored Sep 28, 2018 and Krzysztof Gogolewski committed Sep 28, 2018

Summary:
I noticed while playing around with
https://github.com/fbsamples/ghc-hotswap/ that the main binary needs to
have a custom main function to set `config.keep_cafs = true` when
initialising the runtime. This is pretty annoying, it means an extra
C file with some cryptic incantations in it, and a `-no-hs-main` flag.

So I've replaced this with a link-time flag to GHC, `-fkeep-cafs` that
does the same thing.

Test Plan:
New unit test that tests for the RTS's GC'd CAFs assertion, and also
the -keep-cafs flag.

Reviewers: bgamari, osa1, erikd, noamz

Reviewed By: osa1

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5183