FIR: Fix interface delegation ABI

- Unifies the name of the `$$delegate_<n>` field between K1 and K2.
- Make the `$$delegate_<n>` field private

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/descriptors/IrDelegateDescriptor.kt

@@ -21,6 +21,7 @@ import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.descriptors.impl.PropertyDescriptorImpl
 import org.jetbrains.kotlin.descriptors.impl.VariableDescriptorImpl
 import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.name.SpecialNames
 import org.jetbrains.kotlin.resolve.constants.ConstantValue
 import org.jetbrains.kotlin.types.KotlinType
 import org.jetbrains.kotlin.types.TypeSubstitutor
@@ -97,7 +98,7 @@ class IrPropertyDelegateDescriptorImpl(
 ) :
     IrDelegateDescriptorBase(
         correspondingProperty.containingDeclaration,
-        getDelegateName(correspondingProperty.name),
+        SpecialNames.propertyDelegateName(correspondingProperty.name),
         delegateType,
         correspondingProperty.delegateField?.annotations ?: Annotations.EMPTY
     ),
@@ -111,14 +112,11 @@ class IrImplementingDelegateDescriptorImpl(
 ) :
     IrDelegateDescriptorBase(
         containingDeclaration,
-        Name.identifier("\$\$delegate_$number"),
+        SpecialNames.delegateFieldName(number),
         delegateType
     ),
     IrImplementingDelegateDescriptor
 
-internal fun getDelegateName(name: Name): Name =
-    Name.identifier(name.asString() + "\$delegate")
-
 class IrLocalDelegatedPropertyDelegateDescriptorImpl(
     override val correspondingLocalProperty: VariableDescriptorWithAccessors,
     delegateType: KotlinType,
@@ -127,16 +125,16 @@ class IrLocalDelegatedPropertyDelegateDescriptorImpl(
     VariableDescriptorImpl(
         correspondingLocalProperty.containingDeclaration,
         Annotations.EMPTY,
-        getDelegateName(correspondingLocalProperty.name),
+        SpecialNames.propertyDelegateName(correspondingLocalProperty.name),
         delegateType,
-        org.jetbrains.kotlin.descriptors.SourceElement.NO_SOURCE
+        SourceElement.NO_SOURCE
     ) {
 
     override fun getCompileTimeInitializer(): ConstantValue<*>? = null
     override fun cleanCompileTimeInitializerCache() {}
     override fun isVar(): Boolean = false
     override fun isLateInit(): Boolean = false
-    override fun substitute(substitutor: TypeSubstitutor): VariableDescriptor? = throw UnsupportedOperationException()
+    override fun substitute(substitutor: TypeSubstitutor): VariableDescriptor = throw UnsupportedOperationException()
     override fun getVisibility(): DescriptorVisibility = DescriptorVisibilities.LOCAL
 
     override fun <R : Any?, D : Any?> accept(visitor: DeclarationDescriptorVisitor<R, D>, data: D): R =