hscSimpleIface is returning a bool for whether there were _no changes_
in the iface file. The same bool is called "no_change_at_all" in
mkIface_, and "no_change" in hscWriteIface and other functions. However
it is called "changed" in HscMain.finish and hscMaybeWriteIface, which
is confusing because "changed" and "no_change" have opposite meanings.
This patch renames "changed" to "no_change" to fix this.

Reviewers: simonpj, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5416