Native: fix cinterop for Swift-produced Objective-C categories

For some reason, libclang's indexer doesn't index categories with
__attribute__((external_source_symbol(language="Swift",...))),
so we have to additionally enumerate them explicitly.

^KT-49455 Fixed

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

@@ -988,6 +988,13 @@ public open class NativeIndexImpl(val library: NativeLibrary, val verbose: Boole
             getObjCProtocolAt(cursor)
         }
     }
+
+    fun indexObjCCategory(cursor: CValue<CXCursor>) {
+        if (isAvailable(cursor)) {
+            getObjCCategoryAt(cursor)
+        }
+    }
+
     protected open fun String.isUnknownTemplate() = false
 
     private fun getParentName(cursor: CValue<CXCursor>, pkg: List<String> = emptyList()) : String? {
@@ -1225,6 +1232,13 @@ private fun indexDeclarations(nativeIndex: NativeIndexImpl): CompilationWithPCH
                     when (cursor.kind) {
                         CXCursorKind.CXCursor_ObjCInterfaceDecl -> nativeIndex.indexObjCClass(cursor)
                         CXCursorKind.CXCursor_ObjCProtocolDecl -> nativeIndex.indexObjCProtocol(cursor)
+                        CXCursorKind.CXCursor_ObjCCategoryDecl -> {
+                            // This fixes https://youtrack.jetbrains.com/issue/KT-49455, which effectively seems to be a bug in libclang:
+                            // the libclang indexer doesn't properly index categories with
+                            // `__attribute__((external_source_symbol(language="Swift",...)))`.
+                            // As a workaround, additionally enumerate all the categories explicitly.
+                            nativeIndex.indexObjCCategory(cursor)
+                        }
                         else -> {}
                     }
                 }

kotlin-native/backend.native/tests/interop/objc/tests/kt49455.h

@@ -0,0 +1,17 @@
+#import <Foundation/NSObject.h>
+
+// https://youtrack.jetbrains.com/issue/KT-49455
+
+@interface KT49455 : NSObject
+@end;
+
+__attribute__((external_source_symbol(language="Swift", defined_in="sample",generated_declaration)))
+@interface KT49455 (KT49455Ext)
+- (int)extensionFunction;
+@end
+
+// Just to ensure that unavailable categories don't break anything.
+__attribute__((unavailable("unavailableExtensionFunction is unavailable")))
+@interface KT49455 (KT49455UnavailableExt)
+- (int)unavailableExtensionFunction;
+@end

kotlin-native/backend.native/tests/interop/objc/tests/kt49455.kt

@@ -0,0 +1,7 @@
+import kotlin.test.*
+import objcTests.*
+
+@Test
+fun testKT49455() {
+    assertEquals(111, KT49455().extensionFunction())
+}

kotlin-native/backend.native/tests/interop/objc/tests/kt49455.m

@@ -0,0 +1,10 @@
+#import "kt49455.h"
+
+@implementation KT49455
+@end;
+
+@implementation KT49455 (KT49455Ext)
+- (int)extensionFunction {
+    return 111;
+}
+@end