Instead of pulling a token and looking for `'('` or `')'`,
just look for the character itself. This prevents us from
lexing every single item twice, once to see if it's a
left parenthesis and once to actually parse it.

Partially fixes #12665

Test Plan: Validate

Reviewers: austin, bgamari, hvr

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2623

GHC Trac Issues: #12665