K2: fix signature computation for Objective-C constructors

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/ObjCInterop.kt

@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.descriptors.annotations.AnnotationDescriptor
 import org.jetbrains.kotlin.incremental.components.NoLookupLocation
 import org.jetbrains.kotlin.ir.declarations.*
+import org.jetbrains.kotlin.ir.descriptors.IrBasedClassConstructorDescriptor
 import org.jetbrains.kotlin.ir.expressions.IrConstructorCall
 import org.jetbrains.kotlin.ir.types.classOrNull
 import org.jetbrains.kotlin.ir.types.getPublicSignature
@@ -272,6 +273,12 @@ fun IrConstructor.getObjCInitMethod(): IrSimpleFunction? {
 }
 
 fun ConstructorDescriptor.getObjCInitMethod(): FunctionDescriptor? {
+    if (this is IrBasedClassConstructorDescriptor) {
+        // E.g. in case of K2.
+        // The constructedClass has empty member scope, so we have to delegate to IR to find the init method.
+        return this.owner.getObjCInitMethod()?.descriptor
+    }
+
     return this.annotations.findAnnotation(objCConstructorFqName)?.let {
         val initSelector = it.getAnnotationStringValue("initSelector")
         val memberScope = constructedClass.unsubstitutedMemberScope