[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

kotlin-native/Interop/Indexer/src/main/kotlin/org/jetbrains/kotlin/native/interop/indexer/Indexer.kt

@@ -936,6 +936,11 @@ public open class NativeIndexImpl(val library: NativeLibrary, val verbose: Boole
 
             CXIdxEntity_ObjCClass -> if (cursor.kind != CXCursorKind.CXCursor_ObjCClassRef /* not a forward declaration */) {
                 indexObjCClass(cursor)
+            } else {
+                // It is a class reference. To get the declaration cursor, we can use clang_getCursorReferenced.
+                // If there is a real declaration besides this forward declaration, the function will automatically
+                // resolve it.
+                indexObjCClass(clang_getCursorReferenced(cursor))
             }
 
             CXIdxEntity_ObjCCategory -> {
@@ -946,6 +951,11 @@ public open class NativeIndexImpl(val library: NativeLibrary, val verbose: Boole
 
             CXIdxEntity_ObjCProtocol -> if (cursor.kind != CXCursorKind.CXCursor_ObjCProtocolRef /* not a forward declaration */) {
                 indexObjCProtocol(cursor)
+            } else {
+                // It is a protocol reference. To get the declaration cursor, we can use clang_getCursorReferenced.
+                // If there is a real declaration besides this forward declaration, the function will automatically
+                // resolve it.
+                indexObjCProtocol(clang_getCursorReferenced(cursor))
             }
 
             CXIdxEntity_ObjCProperty -> {