Change overriddenDescriptors' type from Set to Collection

So that the easiest way of constructing a HashSet in the implementation will be
not the only possible way to implement getOverriddenDescriptors(). If there's
not too many overridden descriptors, an implementation may decide to return a
small list for example instead

idea/src/org/jetbrains/kotlin/idea/refactoring/CallableRefactoring.kt

@@ -59,7 +59,7 @@ public abstract class CallableRefactoring<T: CallableDescriptor>(
         return false
     }
 
-    private fun getClosestModifiableDescriptors(): Set<CallableDescriptor> {
+    private fun getClosestModifiableDescriptors(): Collection<CallableDescriptor> {
         return when (kind) {
             DECLARATION -> {
                 Collections.singleton(callableDescriptor)