FIR: Fix interface delegation ABI

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

compiler/testData/ir/irText/firProblems/DelegationAndInheritanceFromJava.fir.kt.txt

@@ -6,51 +6,51 @@ class Impl : A, B {
   }
 
   override fun add(element: @FlexibleNullability String?): Boolean {
-    return <this>.#<$$delegate_0>.add(element = element)
+    return <this>.#$$delegate_0.add(element = element)
   }
 
   override fun addAll(elements: Collection<@FlexibleNullability String?>): Boolean {
-    return <this>.#<$$delegate_0>.addAll(elements = elements)
+    return <this>.#$$delegate_0.addAll(elements = elements)
   }
 
   override fun clear() {
-    <this>.#<$$delegate_0>.clear()
+    <this>.#$$delegate_0.clear()
   }
 
   override operator fun iterator(): MutableIterator<@FlexibleNullability String?> {
-    return <this>.#<$$delegate_0>.iterator()
+    return <this>.#$$delegate_0.iterator()
   }
 
   override fun remove(element: @FlexibleNullability String?): Boolean {
-    return <this>.#<$$delegate_0>.remove(element = element)
+    return <this>.#$$delegate_0.remove(element = element)
   }
 
   override fun removeAll(elements: Collection<@FlexibleNullability String?>): Boolean {
-    return <this>.#<$$delegate_0>.removeAll(elements = elements)
+    return <this>.#$$delegate_0.removeAll(elements = elements)
   }
 
   override fun retainAll(elements: Collection<@FlexibleNullability String?>): Boolean {
-    return <this>.#<$$delegate_0>.retainAll(elements = elements)
+    return <this>.#$$delegate_0.retainAll(elements = elements)
   }
 
   override operator fun contains(element: @FlexibleNullability String?): Boolean {
-    return <this>.#<$$delegate_0>.contains(element = element)
+    return <this>.#$$delegate_0.contains(element = element)
   }
 
   override fun containsAll(elements: Collection<@FlexibleNullability String?>): Boolean {
-    return <this>.#<$$delegate_0>.containsAll(elements = elements)
+    return <this>.#$$delegate_0.containsAll(elements = elements)
   }
 
   override fun isEmpty(): Boolean {
-    return <this>.#<$$delegate_0>.isEmpty()
+    return <this>.#$$delegate_0.isEmpty()
   }
 
   override val size: Int
     override get(): Int {
-      return <this>.#<$$delegate_0>.<get-size>()
+      return <this>.#$$delegate_0.<get-size>()
     }
 
-  local /* final field */ val <$$delegate_0>: B = b
+  private /* final field */ val $$delegate_0: B = b
 
 }