[K/N] cinterop: ignore CXIdxEntity_CXXClass in ObjC header

Unexpected behavior is caused by `@class` declaration in ObjC context.
This entity is not used in CPP context either.

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

@@ -848,7 +848,7 @@ public open class NativeIndexImpl(val library: NativeLibrary, val verbose: Boole
         }
 
         when (kind) {
-            CXIdxEntity_Struct, CXIdxEntity_Union, CXIdxEntity_CXXClass -> {
+            CXIdxEntity_Struct, CXIdxEntity_Union -> {
                 if (entityName == null) {
                     // Skip anonymous struct.
                     // (It gets included anyway if used as a named field type).

kotlin-native/backend.native/tests/interop/objc/tests/kt48491.h

@@ -0,0 +1,5 @@
+@class KT48491_Foo;
+
+__attribute__((external_source_symbol(language="Swift", defined_in="KT48491_Bar",generated_declaration)))
+@interface KT48491_Foo
+@end