FIR: Fix interface delegation ABI

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

core/compiler.common/src/org/jetbrains/kotlin/name/SpecialNames.kt

@@ -101,4 +101,14 @@ object SpecialNames {
     fun isSafeIdentifier(name: Name): Boolean {
         return name.asString().isNotEmpty() && !name.isSpecial
     }
+
+    @JvmStatic
+    fun delegateFieldName(index: Int): Name {
+        return Name.identifier("\$\$delegate_$index")
+    }
+
+    @JvmStatic
+    fun propertyDelegateName(propertyName: Name): Name {
+        return Name.identifier("${propertyName.asString()}\$delegate")
+    }
 }