Correctly reference deserialization constructor added by plugin

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/472

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/GeneratorHelpers.kt

@@ -579,6 +579,6 @@ interface IrBuilderExtension {
         }
     }
 
-    fun IrClass.serializableSyntheticConstructor(): IrConstructorSymbol =
-        this.constructors.single { it.descriptor.isSerializationCtor() }.symbol
+    fun ReferenceSymbolTable.serializableSyntheticConstructor(forClass: ClassDescriptor): IrConstructorSymbol =
+        referenceConstructor(forClass.constructors.single { it.isSerializationCtor() })
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializableIrGenerator.kt

@@ -127,7 +127,7 @@ class SerializableIrGenerator(
         propertiesStart: Int
     ): Int {
         check(superClass.isInternalSerializable)
-        val superCtorRef = compilerContext.externalSymbols.referenceClass(superClass).owner.serializableSyntheticConstructor()
+        val superCtorRef = compilerContext.externalSymbols.serializableSyntheticConstructor(superClass)
         val superProperties = SerializableProperties(superClass, bindingContext).serializableProperties
         val superSlots = superProperties.bitMaskSlotCount()
         val arguments = allValueParameters.subList(0, superSlots) +

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializerIrGenerator.kt

@@ -431,7 +431,7 @@ class SerializerIrGenerator(val irClass: IrClass, override val compilerContext:
         val typeArgs = (loadFunc.returnType as IrSimpleType).arguments.map { (it as IrTypeProjection).type }
         val ctor: IrConstructorSymbol = if (serializableDescriptor.isInternalSerializable) {
             args = bitMasks.map { irGet(it) } + args + irNull()
-            serializableIrClass.serializableSyntheticConstructor()
+            compilerContext.externalSymbols.serializableSyntheticConstructor(serializableDescriptor)
         } else {
             compilerContext.externalSymbols.referenceConstructor(serializableDescriptor.unsubstitutedPrimaryConstructor!!)
         }