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)
+}