Use correct source spans for EmptyCase

Summary:
The parser's calculation of source spans for `EmptyCase`
expressions was a bit off, leading to some wonky-looking error
messages. Easily fixed with some uses of `comb3` and `sLL`.

Test Plan: make test TEST=T15139

Reviewers: bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie, mpickering, carter

GHC Trac Issues: #15139

Differential Revision: https://phabricator.haskell.org/D4685