[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/diagnostics/redundantModifier.fir.txt

@@ -6,7 +6,7 @@ FILE: redundantModifier.kt
 
     }
     public sealed class B : R|kotlin/Any| {
-        private constructor(): R|B| {
+        protected constructor(): R|B| {
             super<R|kotlin/Any|>()
         }