Declare real external symbols for overriden by synthetic function

instead of unbound

Unbound symbols were throwing an exception on attempt to analyze them
in the other parts of compiler and are generally incorrect

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

@@ -56,11 +56,16 @@ interface IrBuilderExtension {
 
     val BackendContext.localSymbolTable: SymbolTable
 
+    private fun IrClass.declareSimpleFunctionWithExternalOverrides(descriptor: FunctionDescriptor): IrSimpleFunction {
+        return compilerContext.localSymbolTable.declareSimpleFunction(startOffset, endOffset, SERIALIZABLE_PLUGIN_ORIGIN, descriptor).also {f ->
+            descriptor.overriddenDescriptors.mapTo(f.overriddenSymbols) {
+                compilerContext.externalSymbols.referenceSimpleFunction(it.original)
+            }
+        }
+    }
+
     fun IrClass.contributeFunction(descriptor: FunctionDescriptor, fromStubs: Boolean = false, bodyGen: IrBlockBodyBuilder.(IrFunction) -> Unit) {
-        val f: IrSimpleFunction = if (!fromStubs) compilerContext.localSymbolTable.declareSimpleFunctionWithOverrides(
-            this.startOffset,
-            this.endOffset,
-            SERIALIZABLE_PLUGIN_ORIGIN,
+        val f: IrSimpleFunction = if (!fromStubs) declareSimpleFunctionWithExternalOverrides(
             descriptor
         ) else compilerContext.externalSymbols.referenceSimpleFunction(descriptor).owner
         f.parent = this