FIR: Fix interface delegation ABI

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

compiler/fir/analysis-tests/testData/resolve/inlineClasses/inlineClassDeclaration.fir.txt

@@ -103,7 +103,7 @@ FILE: inlineClassDeclaration.kt
             super<R|kotlin/Any|>()
         }
 
-        local final field <$$delegate_0>: R|kotlin/Comparable<kotlin/Int>| = R|<local>/x|
+        private final field $$delegate_0: R|kotlin/Comparable<kotlin/Int>| = R|<local>/x|
 
         public final val x: R|kotlin/Int| = R|<local>/x|
             public get(): R|kotlin/Int|