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

compiler/testData/diagnostics/tests/sealed/kt44861.kt

@@ -0,0 +1,17 @@
+// ISSUE: KT-44861
+// DIAGNOSTICS: -UNUSED_VARIABLE
+
+sealed class Foo() {
+    class A : Foo()
+    class B : Foo()
+}
+
+fun Foo(kind: String = "A"): Foo = when (kind) {
+    "A" -> Foo.A()
+    "B" -> Foo.B()
+    else -> throw Exception()
+}
+
+fun main() {
+    val foo = Foo()
+}