Make $default methods non-private, avoid generating accessors for them

#KT-5786 Fixed

compiler/testData/codegen/box/defaultArguments/private/memberFunction.kt

@@ -0,0 +1,11 @@
+// KT-5786 NoSuchMethodError: no accessor for private fun with default arguments
+
+class A {
+    private fun foo(result: String = "OK"): String = result
+
+    companion object {
+        fun bar() = A().foo()
+    }
+}
+
+fun box() = A.bar()