Create folder if missing for .hie files

Summary:
This matches the existing behaviour for .hi files: if the user requests
the interface file be written in some location, we should create the
parent folder if it doesn't already exist.

Reviewers: bgamari, sjakobi

Reviewed By: sjakobi

Subscribers: sjakobi, rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5463