Reported as #13050. Since holes are expressions but not identifiers,
holes were not allowed in infix operator position. This patch introduces
a new production in infix operator parser to allow this.

Reviewers: simonpj, austin, bgamari

Reviewed By: simonpj

Subscribers: simonpj, RyanGlScott, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2910

GHC Trac Issues: #13050