[FE] Make constructors of sealed classes protected instead of internal

compiler/testData/diagnostics/tests/multiplatform/headerClass/extendExpectedClassWithAbstractMember.txt

@@ -70,7 +70,7 @@ public expect interface BaseE {
 }
 
 public sealed class BaseEImpl : BaseE {
-    internal constructor BaseEImpl()
+    protected constructor BaseEImpl()
     public final fun bar(): kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public abstract expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit

compiler/testData/diagnostics/tests/multiplatform/sealedTypeAlias.txt

@@ -23,7 +23,7 @@ public sealed expect class Presence {
 package
 
 public sealed class P {
-    internal constructor P()
+    protected constructor P()
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String

compiler/testData/diagnostics/tests/multiplatform/sealedTypeAliasTopLevel.txt

@@ -37,7 +37,7 @@ public actual object Online : P {
 }
 
 public sealed class P {
-    internal constructor P()
+    protected constructor P()
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String