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/boxWithStdlib/privateConstructor/synthetic.kt

@@ -0,0 +1,26 @@
+// private constructors are transformed into synthetic
+class PrivateConstructor private constructor() {
+    class Nested { val a = PrivateConstructor() }
+}
+
+fun check(klass: Class<*>) {
+    var hasSynthetic = false
+    var hasSimple = false
+    for (method in klass.getDeclaredConstructors()) {
+        if (method.isSynthetic()) {
+            hasSynthetic = true
+        }
+        else {
+            hasSimple = true
+        }
+    }
+    if (hasSynthetic && hasSimple) return
+    throw AssertionError("Class should have both synthetic and non-synthetic constructor: ($hasSynthetic, $hasSimple)")
+}
+
+fun box(): String {
+    check(javaClass<PrivateConstructor>())
+    // Also check that synthetic accessors really work
+    PrivateConstructor.Nested()
+    return "OK"
+}