[KxSerialization] Change visibility for synthetic functions

- synthetic constructor became internal for final classes
- write$Self became internal for final classes

Resolves https://github.com/Kotlin/kotlinx.serialization/issues/2209

Merge-request: KT-MR-9068
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

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

@@ -1,4 +1,4 @@
-// CURIOUS_ABOUT: serialize, deserialize, write$Self, childSerializers, <init>, <clinit>, getDescriptor
+// CURIOUS_ABOUT: serialize, deserialize, write$Self, write$Self$main, childSerializers, <init>, <clinit>, getDescriptor
 // WITH_STDLIB
 
 import kotlinx.serialization.*