[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/smartcasts/lambdas/lambdaInWhenBranch.fir.txt

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