Option for LINE pragmas to get lexed into tokens

This adds a parser-level switch to have 'LINE' and 'COLUMN'
pragmas lexed into actual tokens (as opposed to updating the
position information in the parser).

'lexTokenStream' is the only place where this option is enabled.

Reviewers: bgamari, alexbiehl, mpickering

Reviewed By: mpickering

Subscribers: alanz, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4336

(cherry picked from commit 9a57cfeb)