IR: never attach return type to signatures of fake overrides

Cases that necessitate the return type hack (see KT-46042) always
involve exactly two methods, exactly one of which is a fake override. It
is sufficient to mark one of them.

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/signature/IdSignatureSerializer.kt

@@ -103,7 +103,8 @@ class PublicIdSignatureComputer(val mangler: KotlinMangler.IrMangler) : IdSignat
         }
 
         private val IrDeclarationWithVisibility.isTopLevelPrivate: Boolean
-            get() = visibility == DescriptorVisibilities.PRIVATE && !checkIfPlatformSpecificExport() && parent is IrPackageFragment
+            get() = visibility == DescriptorVisibilities.PRIVATE && !checkIfPlatformSpecificExport() &&
+                    (parent is IrPackageFragment || parent.isFacadeClass)
 
         override fun visitClass(declaration: IrClass) {
             collectParents(declaration)