Improve line numbers handling in serialization intrinsic

Fix nullable serializers on top-level & method with incorrect receiver

Improve handling of different runtime versions

plugins/kotlinx-serialization/testData/codegen/Intrinsics.kt

@@ -40,4 +40,6 @@ fun test() {
     serializer<SerializableObject>()
 
     listSer<List<Box<Int>>>()
+
+    serializer<Simple?>()
 }