[IR] Mark IrClassImpl with an opt-in annotation

KT-59318

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrClassImpl.kt

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.ir.declarations.impl
 
 import org.jetbrains.kotlin.descriptors.*
+import org.jetbrains.kotlin.ir.IrImplementationDetail
 import org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI
 import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.expressions.IrConstructorCall
@@ -15,7 +16,7 @@ import org.jetbrains.kotlin.ir.types.IrSimpleType
 import org.jetbrains.kotlin.ir.types.IrType
 import org.jetbrains.kotlin.name.Name
 
-open class IrClassImpl(
+open class IrClassImpl @IrImplementationDetail constructor(
     override val startOffset: Int,
     override val endOffset: Int,
     override var origin: IrDeclarationOrigin,

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/types/impl/IrErrorClassImpl.kt

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.ir.types.impl
 
 import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.ir.IrFileEntry
+import org.jetbrains.kotlin.ir.IrImplementationDetail
 import org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI
 import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
 import org.jetbrains.kotlin.ir.declarations.*
@@ -19,6 +20,8 @@ import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.utils.addToStdlib.shouldNotBeCalled
 
+
+@OptIn(IrImplementationDetail::class)
 object IrErrorClassImpl : IrClassImpl(
     UNDEFINED_OFFSET, UNDEFINED_OFFSET, IrDeclarationOrigin.ERROR_CLASS, IrClassSymbolImpl(),
     Name.special("<error>"), ClassKind.CLASS, DescriptorVisibilities.DEFAULT_VISIBILITY, Modality.FINAL