[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/fir/analysis-tests/testData/resolve/expresssions/classifierAccessFromCompanion.fir.txt

@@ -5,7 +5,7 @@ FILE: classifierAccessFromCompanion.kt
         }
 
         public sealed class Function : R|kotlin/Any| {
-            private constructor(): R|Factory.Function| {
+            protected constructor(): R|Factory.Function| {
                 super<R|kotlin/Any|>()
             }