ClangImporter: Fix init kind computation with overridden constructors
We used to compute the init kind from the overridden declaration;
this was switched to use the base declaration in 42f72cb0d.
Refactor the init kind computation a little to get the old behavior
back. Otherwise, if a class defines an initializer named -init, we
always import it as designated by virtue of overridding NSObject's
-init.
Fixes <rdar://problem/56674158>.