[K/N] Make cinterop include unused Objective-C forward declarations

Previously, when an Objective-C library had an unused Objective-C
forward declaration (`@class` or `@protocol`), cinterop tool didn't
include it into the resulting klib at all.

This led to a subtle bug (KT-64105). One Obj-C library has unused
Obj-C forward declaration, and another one depends on the first and
uses this forward declaration, e.g. as a function result type.
When building the first cinterop klib, this forward declaration is not
added to `includedForwardDeclarations` in the klib manifest (the
compiler uses this property to decide whether to synthesize the
corresponding class).
When building the second cinterop klib, the forward declaration is not
added to its manifest either, because it is located in the dependency
(and therefore should've been included there).
As a result, the forward declaration is included nowhere, and any
attempt to use it in Kotlin fails, including calling the function from
the second lib.

This commit fixes this bug by including even unused Objective-C forward
declarations, which is consistent with any other kind of declarations
and seems more natural.

^KT-64105 Fixed

compiler/testData/codegen/box/cinterop/kt64105.kt

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+// TARGET_BACKEND: NATIVE
+// MODULE: lib1
+// FILE: lib1.def
+language=Objective-C
+headers=lib1.h
+
+// FILE: lib1.h
+@class Foo;
+@protocol Bar;
+struct Baz;
+
+// MODULE: lib2(lib1)
+// FILE: lib2.def
+language=Objective-C
+headers=lib2.h
+
+// FILE: lib2.h
+#import "../lib1/lib1.h"
+
+Foo* createFoo() {
+    return 0;
+}
+
+id<Bar> createBar() {
+    return 0;
+}
+
+struct Baz* createBaz() {
+    return 0;
+}
+
+// MODULE: main(lib1,lib2)
+// FILE: main.kt
+import kotlinx.cinterop.CPointer
+import lib1.*
+import lib2.*
+import cnames.structs.Baz
+import objcnames.classes.Foo
+import objcnames.protocols.BarProtocol
+
+@OptIn(kotlinx.cinterop.ExperimentalForeignApi::class)
+fun box(): String {
+    val foo: Foo? = createFoo()
+    if (foo !== null) return "FAIL 1"
+
+    val bar: BarProtocol? = createBar()
+    if (bar !== null) return "FAIL 2"
+
+    val baz: CPointer<Baz>? = createBaz()
+    if (baz !== null) return "FAIL 3"
+
+    return "OK"
+}