[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/recursiveCallOnWhenWithSealedClass.fir.txt

@@ -1,6 +1,6 @@
 FILE: recursiveCallOnWhenWithSealedClass.kt
     public sealed class Maybe<T> : R|kotlin/Any| {
-        private constructor<T>(): R|Maybe<T>| {
+        protected constructor<T>(): R|Maybe<T>| {
             super<R|kotlin/Any|>()
         }