Private constructors are now accessed via synthetic constructor with DEFAULT_CONSTRUCTOR_MARKER as an additional argument #KT-6299 Fixed

A set of tests provided. Some external tests fixed accordingly.
Companion object creation changed accordingly.
Derived classes now can use base class with the private constructor.
Refactoring of AccessorForFunctionDescriptor.

compiler/testData/codegen/box/privateConstructors/withLinkedObjects.kt

@@ -0,0 +1,13 @@
+// See also KT-6299
+public open class Outer private constructor(val p: Outer?) {
+    object Inner: Outer(null)
+    object Other: Outer(Inner)
+    object Another: Outer(Other)
+}
+
+fun box(): String {
+    val outer = Outer.Inner
+    val other = Outer.Other
+    val another = Outer.Another
+    return "OK"
+}