Output source notes in extended DWARF DIEs

Authored by bgamari.

Description

Output source notes in extended DWARF DIEs

In order to accomplish this we need to ensure that emit DIEs for all
DebugBlocks, even those that have been optimized out, lest we end up
with undefined symbols of parents at link time.

Differential Revision: https://phabricator.haskell.org/D1279