[FE] Prohibit sealed fun interfaces

#KT-44947 Fixed

compiler/testData/diagnostics/tests/sealed/interfaces/sealedFunInterface.fir.kt

@@ -0,0 +1,15 @@
+sealed fun interface A { // error
+    fun foo()
+}
+
+sealed interface Base {
+    sealed fun interface Derived : Base {  // error
+        fun foo()
+    }
+}
+
+sealed interface IBase {
+    fun interface IDerived : IBase { // OK
+        fun foo()
+    }
+}

compiler/testData/diagnostics/tests/sealed/interfaces/sealedFunInterface.kt

@@ -0,0 +1,15 @@
+<!UNSUPPORTED!>sealed<!> fun interface A { // error
+    fun foo()
+}
+
+sealed interface Base {
+    <!UNSUPPORTED!>sealed<!> fun interface Derived : Base {  // error
+        fun foo()
+    }
+}
+
+sealed interface IBase {
+    fun interface IDerived : IBase { // OK
+        fun foo()
+    }
+}

compiler/testData/diagnostics/tests/sealed/interfaces/sealedFunInterface.txt

@@ -0,0 +1,34 @@
+package
+
+public sealed fun interface A {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public sealed interface Base {
+    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
+
+    public sealed fun interface Derived : Base {
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public abstract fun foo(): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
+
+public sealed interface IBase {
+    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
+
+    public fun interface IDerived : IBase {
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public abstract fun foo(): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}