Summary:
setProgramDynFlags invalidates the whole module graph, forcing
everything to be re-summarised (including preprocessing) on every
:reload.

Looks like this was a bad regression in 8.0, but we didn't notice
because there was no test for it.  Now there is!

Test Plan:
* validate
* new unit test

Reviewers: bgamari, triple, austin, niteria, erikd, jme

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3398