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/bytecodeText/staticFields/classObject.kt

@@ -4,4 +4,4 @@ class A {
     }
 }
 // A and companion object constructor call
-// 2 ALOAD 0
+// 3 ALOAD 0

compiler/testData/codegen/bytecodeText/staticFields/classObjectSyntheticAccessor.kt

@@ -4,5 +4,5 @@ class A {
     }
 }
 // A and companion object constructor call
-// 2 ALOAD 0
+// 3 ALOAD 0
 // 1 synthetic access\$getR