Correctly determine the type of serializable property

when supertype of serializable class is generic and also serializable,
and contains the property with type with its generic parameter.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/1264
#KT-43910 Fixed
#KT-49660 Fixed

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

@@ -0,0 +1,56 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+@Serializable
+abstract class Top<T: Any> {
+    var top: T? = null
+}
+
+@Serializable
+open class Intermediate<V>: Top<List<V>>() {
+    var inter: V? = null
+    override fun toString(): String {
+        return "Intermediate($top, $inter)"
+    }
+}
+
+@Serializable
+open class Bottom: Intermediate<String>() {
+    var bot: String? = null
+    override fun toString(): String {
+        return "Bottom($top, $inter, $bot)"
+    }
+}
+
+@Serializable
+class Bottom2: Bottom() {
+    override fun toString(): String {
+        return "Bottom2($top, $inter, $bot)"
+    }
+}
+
+@Serializable
+data class Full(
+    val b: Bottom2,
+    val i: Intermediate<String>
+)
+
+fun box(): String {
+    val j = Json { ignoreUnknownKeys = true }
+    val b = Bottom2().apply {
+        top = listOf("a", "b")
+        inter = "v"
+        bot = "bot"
+    }
+    val f = Full(b, b)
+    val encoded = j.encodeToString(f)
+    if (encoded != """{"b":{"top":["a","b"],"inter":"v","bot":"bot"},"i":{"top":["a","b"],"inter":"v"}}""") return "Encoded: $encoded"
+
+    val decoded = j.decodeFromString<Full>(encoded)
+    if (decoded.toString() != "Full(b=Bottom2([a, b], v, bot), i=Intermediate([a, b], v))") return "Decoded: $decoded"
+    return "OK"
+}