Allow open impl class for final header class

compiler/testData/diagnostics/tests/multiplatform/headerClass/implOpenClass.kt

@@ -0,0 +1,16 @@
+// !LANGUAGE: +MultiPlatformProjects
+// MODULE: m1-common
+// FILE: common.kt
+
+header class Foo
+
+header fun getFoo(): Foo
+
+// MODULE: m2-jvm(m1-common)
+// FILE: jvm.kt
+
+impl open class Foo
+
+class Bar : Foo()
+
+impl fun getFoo(): Foo = Bar()

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

@@ -0,0 +1,31 @@
+// -- Module: <m1-common> --
+package
+
+public header fun getFoo(): Foo
+
+public final header class Foo {
+    public constructor Foo()
+    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
+}
+
+
+// -- Module: <m2-jvm> --
+package
+
+public impl fun getFoo(): Foo
+
+public final class Bar : Foo {
+    public constructor Bar()
+    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 open impl class Foo {
+    public constructor Foo()
+    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
+}