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/genericBaseClassSimple.kt

@@ -0,0 +1,46 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+// From #1264
+@Serializable
+sealed class TypedSealedClass<T>(val a: T) {
+    @Serializable
+    class Child(val y: Int) : TypedSealedClass<String>("10") {
+        override fun toString(): String = "Child($a, $y)"
+    }
+}
+
+// From #KT-43910
+@Serializable
+open class ValidatableValue<T : Any, V: Any>(
+    var value: T? = null,
+    var error: V? = null,
+)
+
+@Serializable
+class Email<T: Any> : ValidatableValue<String, T>() { // Note this is a different T
+    override fun toString(): String {
+        return "Email($value, $error)"
+    }
+}
+
+fun box(): String {
+    val encodedChild = """{"a":"11","y":42}"""
+    val decodedChild = Json.decodeFromString<TypedSealedClass.Child>(encodedChild)
+    if (decodedChild.toString() != "Child(11, 42)") return "DecodedChild: $decodedChild"
+    Json.encodeToString(decodedChild)?.let { if (it != encodedChild) return "EncodedChild: $it" }
+
+    val email = Email<Int>().apply {
+        value = "foo"
+        error = 1
+    }
+    val encodedEmail = Json.encodeToString(email)
+    if (encodedEmail != """{"value":"foo","error":1}""") return "EncodedEmail: $encodedEmail"
+    val decodedEmail = Json.decodeFromString<Email<Int>>(encodedEmail)
+    if (decodedEmail.toString() != "Email(foo, 1)") return "DecodedEmail: $decodedEmail"
+    return "OK"
+}