[FE] Allow declaring protected constructors in sealed classes

#KT-44865 Fixed

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

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