Summary:
Provide a way to generate {-# LINE #-} pragmas when generating
Decs in Template Haskell. This allows more meaningful line
numbers to be reported in compile-time errors for dynamically
generated code.

Test Plan: Run test suite

Reviewers: austin, hvr

Reviewed By: austin

Subscribers: hvr, simonmar, ezyang, carter, thomie

Differential Revision: https://phabricator.haskell.org/D299