[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/inheritorInDifferentModule.fir.kt

@@ -14,4 +14,4 @@ class A : Base()
 
 package a
 
-class B : <!HIDDEN!>Base<!>()
+class B : Base()