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/classes/implicitNotNullOnDelegatedImplementation.fir.kt.txt

@@ -55,10 +55,10 @@ class TestJFoo : IFoo {
   }
 
   override fun foo(): String {
-    return <this>.#<$$delegate_0>.foo() /*!! String */
+    return <this>.#$$delegate_0.foo() /*!! String */
   }
 
-  local /* final field */ val <$$delegate_0>: JFoo = JFoo()
+  private /* final field */ val $$delegate_0: JFoo = JFoo()
 
 }
 
@@ -70,10 +70,10 @@ class TestK1 : IFoo {
   }
 
   override fun foo(): String {
-    return <this>.#<$$delegate_0>.foo() /*!! String */
+    return <this>.#$$delegate_0.foo() /*!! String */
   }
 
-  local /* final field */ val <$$delegate_0>: K1 = K1()
+  private /* final field */ val $$delegate_0: K1 = K1()
 
 }
 
@@ -85,10 +85,10 @@ class TestK2 : IFoo {
   }
 
   override fun foo(): String {
-    return <this>.#<$$delegate_0>.foo()
+    return <this>.#$$delegate_0.foo()
   }
 
-  local /* final field */ val <$$delegate_0>: K2 = K2()
+  private /* final field */ val $$delegate_0: K2 = K2()
 
 }
 
@@ -100,10 +100,10 @@ class TestK3 : IFoo {
   }
 
   override fun foo(): String {
-    return <this>.#<$$delegate_0>.foo()
+    return <this>.#$$delegate_0.foo()
   }
 
-  local /* final field */ val <$$delegate_0>: K3 = K3()
+  private /* final field */ val $$delegate_0: K3 = K3()
 
 }
 
@@ -115,10 +115,10 @@ class TestK4 : IFoo {
   }
 
   override fun foo(): String {
-    return <this>.#<$$delegate_0>.foo() /*!! String */
+    return <this>.#$$delegate_0.foo() /*!! String */
   }
 
-  local /* final field */ val <$$delegate_0>: K4 = K4()
+  private /* final field */ val $$delegate_0: K4 = K4()
 
 }