Cache the fingerprint of sOpt_P
Before this change we would compute a hash of
all the command line -optP flags once per file.
With a lot of files and many -optP flags, that's a lot
of repeated work.
I added a new Note that explains the approach and rationale.
Test Plan: new test
Reviewers: simonmar, simonpj, bgamari
Reviewed By: simonpj
Subscribers: rwbarton, thomie, carter
GHC Trac Issues: Trac #14697
Differential Revision: https://phabricator.haskell.org/D4445