Correct BangPat SrcSpan calculation

Summary:
Previously when the split was performed in splitBang, `BangPat` was given the
same SrcSpan as the whole of the LHS of the declaration. This patch
correctly calculates the value.

Reviewers: alanz, austin

Reviewed By: alanz, austin

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D1020

GHC Trac Issues: #10588

(cherry picked from commit c6bb2fc5)