Capture original source for literals

Summary:
Make HsLit and OverLitVal have original source strings, for source to
source conversions using the GHC API

This is part of the ongoing AST Annotations work, as captured in
https://ghc.haskell.org/trac/ghc/wiki/GhcAstAnnotations

Test Plan: ./sh validate

Reviewers: simonpj, austin

Subscribers: thomie, goldfire, carter, simonmar

Differential Revision: https://phabricator.haskell.org/D412

GHC Trac Issues: #9628