[IR] Don't produce gaps for skipped IR declarations during serialization

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/IrFileSerializer.kt

@@ -21,8 +21,6 @@ import org.jetbrains.kotlin.ir.visitors.acceptChildrenVoid
 import org.jetbrains.kotlin.ir.visitors.acceptVoid
 import org.jetbrains.kotlin.library.SerializedDeclaration
 import org.jetbrains.kotlin.library.SerializedIrFile
-import org.jetbrains.kotlin.library.SkippedDeclaration
-import org.jetbrains.kotlin.library.TopLevelDeclaration
 import org.jetbrains.kotlin.library.impl.IrMemoryArrayWriter
 import org.jetbrains.kotlin.library.impl.IrMemoryDeclarationWriter
 import org.jetbrains.kotlin.library.impl.IrMemoryStringWriter
@@ -1425,7 +1423,8 @@ open class IrFileSerializer(
 
         file.declarations.forEach {
             if (skipExpects && it.descriptor.isExpectMember && !it.descriptor.isSerializableExpectClass) {
-                topLevelDeclarations.add(SkippedDeclaration)
+                // Skip the declaration unless it is `expect annotation class` marked with `OptionalExpectation`
+                // without the corresponding `actual` counterpart for the current leaf target.
                 return@forEach
             }
 
@@ -1437,7 +1436,7 @@ open class IrFileSerializer(
             // TODO: keep order similar
             val sigIndex = protoIdSignatureMap[idSig]
                 ?: if (it is IrErrorDeclaration) protoIdSignature(idSig) else error("Not found ID for $idSig (${it.render()})")
-            topLevelDeclarations.add(TopLevelDeclaration(sigIndex, idSig.toString(), byteArray))
+            topLevelDeclarations.add(SerializedDeclaration(sigIndex, idSig.toString(), byteArray))
             proto.addDeclarationId(sigIndex)
         }
 

compiler/util-klib/src/org/jetbrains/kotlin/library/KotlinLibraryWriter.kt

@@ -32,24 +32,8 @@ class SerializedMetadata(
     val fragmentNames: List<String>
 )
 
-sealed class SerializedDeclaration {
+class SerializedDeclaration(val id: Int, val declarationName: String, val bytes: ByteArray) {
-    abstract val id: Int
+    val size = bytes.size
-    abstract val size: Int
-    abstract val bytes: ByteArray
-
-    abstract val declarationName: String
-}
-
-class TopLevelDeclaration(override val id: Int, override val declarationName: String, override val bytes: ByteArray) :
-    SerializedDeclaration() {
-    override val size = bytes.size
-}
-
-object SkippedDeclaration : SerializedDeclaration() {
-    override val id = -1
-    override val size = 0
-    override val bytes = ByteArray(0)
-    override val declarationName: String = "<SKIPPED>"
 }
 
 class SerializedIrFile(