Do not create cacheableChildSerializers unless it is necessary

Creation of this property in the SerializerIrGenerator.<init> can lead to
a 'Serializer not found' internal error when generator is applied to a fully-customized external serializer.
In that case, generator is still created, but none of the generateSave/Load functions are called,
so cacheableChildSerializers(Property) is not necessary.

#KT-57730 Fixed
Fixes https://github.com/Kotlin/kotlinx.serialization/issues/2260

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

@@ -25,4 +25,4 @@ object URLSerializer : KSerializer<URL> {
 fun box(): String {
     if (URLSerializer.descriptor.toString() != "PrimitiveDescriptor(java.net.URL)") return URLSerializer.descriptor.toString()
     return "OK"
-}
+}

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

@@ -0,0 +1,30 @@
+// TARGET_BACKEND: JVM_IR
+
+// FULL_JDK
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import java.math.BigDecimal
+
+@Serializable(with = DataBigDecimal.Serializer::class)
+data class DataBigDecimal(val value: BigDecimal) {
+
+    @kotlinx.serialization.Serializer(forClass = DataBigDecimal::class)
+    object Serializer : KSerializer<DataBigDecimal> {
+
+        override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("my.DataBigDecimal", PrimitiveKind.STRING)
+
+        override fun deserialize(decoder: Decoder): DataBigDecimal =
+            TODO()
+
+        override fun serialize(encoder: Encoder, value: DataBigDecimal): Unit =
+            TODO()
+    }
+}
+
+fun box(): String {
+    if (DataBigDecimal.Serializer.descriptor.toString() != "PrimitiveDescriptor(my.DataBigDecimal)") return DataBigDecimal.Serializer.descriptor.toString()
+    return "OK"
+}