Only private constructors for sealed / enum classes #KT-12377 Fixed

Also #KT-8497 Fixed

compiler/testData/diagnostics/tests/sealed/NonPrivateConstructor.kt

@@ -0,0 +1,7 @@
+sealed class Sealed <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>protected<!> constructor(val x: Int) {
+    object FIRST : Sealed()
+
+    <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>public<!> constructor(): this(42)
+
+    constructor(y: Int, z: Int): this(y + z)
+}

compiler/testData/diagnostics/tests/sealed/NonPrivateConstructor.txt

@@ -0,0 +1,19 @@
+package
+
+public sealed class Sealed {
+    public constructor Sealed()
+    protected constructor Sealed(/*0*/ x: kotlin.Int)
+    private constructor Sealed(/*0*/ y: kotlin.Int, /*1*/ z: kotlin.Int)
+    public final val x: kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public object FIRST : Sealed {
+        private constructor FIRST()
+        public final override /*1*/ /*fake_override*/ val x: kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}