[FIR] Fix processing constructors of sealed classes

- Allow declaring protected constructors in sealed classes
- Make default visibility of sealed class constructor `protected`

KT-44861
KT-44865

compiler/testData/diagnostics/tests/sealed/NonPrivateConstructor.fir.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)
 }