Generate private constructors for Enums

#KT-2680 Fixed
  #KT-16867 Fixed

compiler/testData/asJava/lightClasses/DeprecatedEnumEntry.java

@@ -9,5 +9,5 @@ public enum E {
     @kotlin.Deprecated(message = "b")
     Entry3;
 
-    protected E() { /* compiled code */ }
+    private E() { /* compiled code */ }
 }

compiler/testData/codegen/bytecodeListing/inlineClasses/inlineClassTypeParametersInConstructor.txt

@@ -26,7 +26,7 @@ public final enum class TestEnum {
     public final static field ANSWER: TestEnum
     private final field z: int
     static method <clinit>(): void
-    protected method <init>(p0: java.lang.String, p1: int, p2: int): void
+    private method <init>(p0: java.lang.String, p1: int, p2: int): void
     public final method getZ(): int
     public static method valueOf(p0: java.lang.String): TestEnum
     public static method values(): TestEnum[]

compiler/testData/writeFlags/function/constructors/enum.kt

@@ -0,0 +1,7 @@
+enum class Foo {
+    A, B, C
+}
+
+// TESTED_OBJECT_KIND: function
+// TESTED_OBJECTS: Foo, <init>
+// FLAGS: ACC_PRIVATE