[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/exhaustiveness/negative/missingSealedInheritor.fir.txt

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

compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/exhaustiveness_sealedClass.fir.txt

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

compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/exhaustiveness_sealedObject.fir.txt

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

compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/exhaustiveness_sealedSubClass.fir.txt

@@ -1,12 +1,12 @@
 FILE: exhaustiveness_sealedSubClass.kt
     public sealed class A : R|kotlin/Any| {
-        private constructor(): R|A| {
+        protected constructor(): R|A| {
             super<R|kotlin/Any|>()
         }
 
     }
     public sealed class B : R|A| {
-        private constructor(): R|B| {
+        protected constructor(): R|B| {
             super<R|A|>()
         }
 
@@ -18,13 +18,13 @@ FILE: exhaustiveness_sealedSubClass.kt
 
     }
     public sealed class D : R|B| {
-        private constructor(): R|D| {
+        protected constructor(): R|D| {
             super<R|B|>()
         }
 
     }
     public sealed class E : R|B| {
-        private constructor(): R|E| {
+        protected constructor(): R|E| {
             super<R|B|>()
         }