[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/incompatibleModifiers.fir.txt

@@ -30,13 +30,13 @@ FILE: incompatibleModifiers.kt
 
     }
     public final class F : R|kotlin/Any| {
-        private constructor(): R|F| {
+        protected constructor(): R|F| {
             super<R|kotlin/Any|>()
         }
 
     }
     public sealed class G : R|kotlin/Any| {
-        private constructor(): R|G| {
+        protected constructor(): R|G| {
             super<R|kotlin/Any|>()
         }
 
@@ -57,7 +57,7 @@ FILE: incompatibleModifiers.kt
 
     }
     public sealed data class I : R|kotlin/Any| {
-        private constructor(i: R|kotlin/Int|): R|I| {
+        protected constructor(i: R|kotlin/Int|): R|I| {
             super<R|kotlin/Any|>()
         }
 
@@ -126,7 +126,7 @@ FILE: incompatibleModifiers.kt
         }
 
         public sealed inner class Z : R|kotlin/Any| {
-            private constructor(): R|X.Z| {
+            protected constructor(): R|X.Z| {
                 super<R|kotlin/Any|>()
             }
 

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|>()
         }
 

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|

compiler/fir/analysis-tests/testData/resolve/diagnostics/sealedClassConstructorCall.kt

@@ -1,3 +1,3 @@
 sealed class A
 
-val b = <!SEALED_CLASS_CONSTRUCTOR_CALL{LT}!><!SEALED_CLASS_CONSTRUCTOR_CALL{PSI}!>A<!>()<!>
+val b = <!HIDDEN{LT}!><!HIDDEN{PSI}!>A<!>()<!>

compiler/fir/analysis-tests/testData/resolve/diagnostics/sealedSupertype.fir.txt

@@ -1,6 +1,6 @@
 FILE: sealedSupertype.kt
     public sealed class A : R|kotlin/Any| {
-        private constructor(): R|A| {
+        protected constructor(): R|A| {
             super<R|kotlin/Any|>()
         }
 
@@ -22,7 +22,7 @@ FILE: sealedSupertype.kt
 
     }
     public sealed class P : R|kotlin/Any| {
-        private constructor(): R|P| {
+        protected constructor(): R|P| {
             super<R|kotlin/Any|>()
         }