Fix ClassCastException in SamAdapterOverridabilityCondition

This was happening on the upcoming hierarchy of property getters and setters in
kotlin.reflect. No test added because it's not so easy to come up with a small
example, and because the fix itself is rather trivial

compiler/frontend.java/src/org/jetbrains/kotlin/load/java/sam/SamAdapterOverridabilityCondition.java

@@ -30,7 +30,7 @@ import java.util.List;
 public class SamAdapterOverridabilityCondition implements ExternalOverridabilityCondition {
     @Override
     public boolean isOverridable(@NotNull CallableDescriptor superDescriptor, @NotNull CallableDescriptor subDescriptor) {
-        if (subDescriptor instanceof PropertyDescriptor) {
+        if (!(subDescriptor instanceof SimpleFunctionDescriptor)) {
             return true;
         }