Fix #15502 by not casting to Int during TH conversion

Summary:
When turning an `IntegerL` to an `IntegralLit` during TH
conversion, we were stupidly casting an `Integer` to an `Int` in
order to determine how it should be pretty-printed. Unsurprisingly,
this causes problems when the `Integer` doesn't lie within the bounds
of an `Int`, as demonstrated in #15502.

The fix is simple: don't cast to an `Int`.

Test Plan: make test TEST=T15502

Reviewers: bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, carter

GHC Trac Issues: #15502

Differential Revision: https://phabricator.haskell.org/D5089