Generate private methods in TraitImpl as private, don't generate delegation to private trait methods

compiler/testData/codegen/box/traits/noPrivateDelegation.kt

@@ -0,0 +1,16 @@
+interface Z{
+
+    private fun extension(): String {
+        return "OK"
+    }
+}
+
+object Z2 : Z {
+
+}
+
+fun box() : String {
+    val size = Class.forName("Z2").declaredMethods.size
+    if (size != 0) return "fail: $size"
+    return "OK"
+}