Add declarations from serialization plugin to FIR metadata declarations provider.

These declarations should not be visible to users (and therefore are not added to FIR),
but plugin itself can reference them in already compiled serializable classes,
and therefore they should be available in metadata:

- synthetic deserialization constructor
- static write$Self function

See also:
^KT-55885

plugins/kotlinx-serialization/testData/boxIr/multimoduleInheritance.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_K2: JVM_IR
 // TARGET_BACKEND: JVM_IR
 
 // WITH_STDLIB
@@ -18,6 +17,15 @@ open class OpenBody {
 @Serializable
 abstract class AbstractConstructor(var optional: String = "foo")
 
+// TODO: test fails for K2 if places of 'color' and 'name' are swapped
+// because of https://youtrack.jetbrains.com/issue/KT-54792 (KT-20980)
+// and serialization proto extension is not available in K2.
+@Serializable
+open class Vehicle {
+    var color: String? = null
+    var name: String? = null
+}
+
 
 // MODULE: app(lib)
 // FILE: app.kt
@@ -35,6 +43,11 @@ class Test1: OpenBody()
 @Serializable
 class Test2: AbstractConstructor()
 
+@Serializable
+open class Car : Vehicle() {
+    var maxSpeed: Int = 100
+}
+
 fun test1() {
     val string = Json.encodeToString(Test1.serializer(), Test1())
     assertEquals("{}", string)
@@ -49,8 +62,29 @@ fun test2() {
     assertEquals("foo", reconstructed.optional)
 }
 
+fun test3() {
+    val json = Json { allowStructuredMapKeys = true; encodeDefaults = true }
+
+    val car = Car()
+    car.maxSpeed = 100
+    car.name = "ford"
+    val s = json.encodeToString(Car.serializer(), car)
+    assertEquals("""{"color":null,"name":"ford","maxSpeed":100}""", s)
+    val restoredCar = json.decodeFromString(Car.serializer(), s)
+    assertEquals(100, restoredCar.maxSpeed)
+    assertEquals("ford", restoredCar.name)
+    assertEquals(null, restoredCar.color)
+}
+
 fun box(): String {
-    test1()
-    test2()
-    return "OK"
+    try {
+        test1()
+        test2()
+        test3()
+        return "OK"
+    } catch (e: Throwable) {
+        e.printStackTrace()
+        return e.message!!
+    }
+
 }