Generate private interface companion object as package-private synthetic

compiler/testData/codegen/box/objects/companionObjectAccess/kt27121.kt

@@ -0,0 +1,14 @@
+// !LANGUAGE: +ProperVisibilityForCompanionObjectInstanceField
+// IGNORE_BACKEND: JS, JS_IR, JVM_IR
+
+interface A {
+    fun test() = ok()
+
+    private companion object {
+        fun ok() = "OK"
+    }
+}
+
+class C : A
+
+fun box() = C().test()