Fix #14719 by using the setting the right SrcSpan

Currently, error messages that germane to GADT constructors
put the source span at only the first character in the constructor,
leading to insufficient caret diagnostics. This can be easily fixed
by using a source span that spans the entire constructor, instead of
just the first character.

Test Plan: make test TEST=T14719

Reviewers: alanz, bgamari, simonpj

Reviewed By: alanz, simonpj

Subscribers: simonpj, goldfire, rwbarton, thomie, carter

GHC Trac Issues: #14719

Differential Revision: https://phabricator.haskell.org/D4344

(cherry picked from commit 59fa7b32)