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

#KT-5786 Fixed

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

@@ -0,0 +1,16 @@
+var state: String = "Fail"
+
+class A private(x: String = "OK") {
+    init {
+        state = x
+    }
+
+    companion object {
+        fun foo() = A()
+    }
+}
+
+fun box(): String {
+    A.foo()
+    return state
+}