Fix for built-in Natural literals desugaring

Authored by Sylvain Henry <hsyl20@gmail.com> on Fri, Jul 6, 10:01 AM.

Description

Fix for built-in Natural literals desugaring

The recent patch "Built-in Natural literals in Core"
(https://phabricator.haskell.org/rGHCfe770c211631e7b4c9b0b1e88ef9b6046c6
585ef) introduced a regression when desugaring large numbers.

This patch fixes it and adds a regression test.

Reviewers: hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: Trac #15301

Differential Revision: https://phabricator.haskell.org/D4885