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"
+}

compiler/testData/codegen/boxWithStdlib/synthetic/syntheticAccessorNames.kt

@@ -2,10 +2,6 @@
 // This is crucial for some JVM frameworks like Quasar which rely on the bytecode being similar to the one generated by javac
 // See https://youtrack.jetbrains.com/issue/KT-6870
 
-class PrivateConstructor private constructor() {
-    class Nested { val a = PrivateConstructor() }
-}
-
 class PrivatePropertyGet {
     private val x = 42
 
@@ -33,13 +29,11 @@ fun check(klass: Class<*>) {
 }
 
 fun box(): String {
-    check(javaClass<PrivateConstructor>())
     check(javaClass<PrivatePropertyGet>())
     check(javaClass<PrivatePropertySet>())
     check(javaClass<PrivateMethod>())
 
     // Also check that synthetic accessors really work
-    PrivateConstructor.Nested()
     PrivatePropertyGet().Inner()
     PrivatePropertySet().Inner()
     PrivateMethod().Inner()