[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/sealedClassConstructorCall.fir.txt

@@ -1,9 +1,9 @@
 FILE: sealedClassConstructorCall.kt
     public sealed class A : R|kotlin/Any| {
-        private constructor(): R|A| {
+        protected constructor(): R|A| {
             super<R|kotlin/Any|>()
         }
 
     }
-    public final val b: R|A| = R|/A.A|()
+    public final val b: R|A| = <HIDDEN: /A.A is invisible>#()
         public get(): R|A|