Do not warn about unused underscore-prefixed fields (fixes Trac #12609)

Authored by adamgundry on Oct 1 2016, 4:56 PM.

Description

Do not warn about unused underscore-prefixed fields (fixes Trac Trac #12609)

When DuplicateRecordFields is enabled, the mangling of selector names
was causing them to be reported as unused even if prefixed by an
underscore. This corrects the OccName used by the check.

Test Plan: New test overloadedrecflds/should_compile/T12609

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2549

GHC Trac Issues: Trac #12609