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/inner.kt

@@ -0,0 +1,15 @@
+// See also KT-6299
+public open class Outer private constructor(val s: String) {
+    inner class Inner: Outer("O") {
+        fun foo(): String {
+            return this.s + this@Outer.s
+        }
+    }
+    class Nested: Outer("K") 
+    fun bar() = Inner()
+}
+
+fun box(): String {
+    val inner = Outer.Nested().bar()
+    return inner.foo()
+}