[FE] Allow declaring protected constructors in sealed classes

#KT-44865 Fixed

compiler/testData/diagnostics/tests/sealed/protectedConstructors_enabled.fir.kt

@@ -0,0 +1,41 @@
+// LANGUAGE: +AllowSealedInheritorsInDifferentFilesOfSamePackage
+// DIAGNOSTICS: -UNUSED_PARAMETER
+
+sealed class Case1(val x: Int) {
+    <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>protected constructor(s: String) : this(s.length)<!>
+
+    class Inheritor1 : Case1(10)
+    class Inheritor2 : Case1("Hello")
+}
+
+sealed class Case2 <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>protected constructor(val x: Int)<!> { // should be REDUNDANT_MODIFIER
+    <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>protected constructor(s: String) : this(s.length)<!>
+
+    class Inheritor1 : Case2(10)
+    class Inheritor2 : Case2("Hello")
+}
+
+sealed class Case3 private constructor(val x: Int) {
+    <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>protected constructor(s: String) : this(s.length)<!>
+
+    class Inheritor1 : Case3(10) // should OK
+    class Inheritor2 : Case3("Hello")
+}
+
+class Case3Inheritor3 : <!INAPPLICABLE_CANDIDATE!>Case3<!>(20) // should be an error in 1.6 (?)
+
+sealed class Case4 {
+    <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>protected constructor(x: Int)<!>
+    <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>protected constructor(s: String) : this(s.length)<!>
+
+    class Inheritor1 : Case4(10)
+    class Inheritor2 : Case4("Hello")
+}
+
+sealed class Case5() {
+    private constructor(x: Int) : this()
+    <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>protected constructor(x: Byte) : this()<!>
+    <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>internal constructor(x: Short) : this()<!>
+    <!NON_PRIVATE_CONSTRUCTOR_IN_SEALED!>public constructor(x: Long) : this()<!>
+    constructor(x: Double) : this()
+}