[K2 Serialization] Store serializable properties in metadata extension

This solution is temporary (see KT-64694 for details)

^KT-64312 Fixed

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

@@ -0,0 +1,34 @@
+// ISSUE: KT-64312
+// WITH_STDLIB
+// MODULE: lib
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+@Serializable
+abstract class Base {
+    @SerialName("properties")
+    protected var propertiesInternal: String? = null
+
+    val properties: String // should not be serialized because it does not have a backing field
+        get() = propertiesInternal?.reversed().orEmpty()
+}
+
+// MODULE: main(lib)
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+@Serializable
+class Derived: Base() {
+    fun initialize() {
+        propertiesInternal = "KO"
+    }
+}
+
+fun box(): String {
+    val expected = Derived().also { it.initialize() }
+    val json = Json.encodeToString(Derived.serializer(), expected)
+    if (json != """{"properties":"KO"}""") return "Fail: $json"
+    val actual = Json.decodeFromString(Derived.serializer(), json)
+    if (expected.properties != actual.properties) return "Fail: $actual"
+    return actual.properties
+}