Most of the time the name importer does a good job deciding whether to
import a particular method as throwing or not. However, when a method
is an override, it skips all that work and assumes the decisions made
for the superclass method apply here as well---which makes sense,
since you're going to get the subclass implementation if you call the
superclass's entry point. This can really throw things off if the
types /don't/ match up, though. Handle the one case where this is
legal according to the rules of Objective-C, and make sure we don't
import methods in the other cases.
rdar://problem/30705461