Back-end: write correct visibility flags for private inner classes

compiler/testData/writeFlags/class/visibility/private/class.kt

@@ -0,0 +1,7 @@
+//ACC_PUBLIC because private in kotlin is package private in java + it should be visible in subpackages
+private class MyClass() {
+}
+
+// TESTED_OBJECT_KIND: class
+// TESTED_OBJECTS: MyClass
+// FLAGS: ACC_FINAL, ACC_PUBLIC, ACC_SUPER