Implement NSDate as a small object (tagged pointer) - clang/libobjc2 only (#451)

* Implement GSSmallObject Class

* Remove private concrete class access

* Change secondary bias

* NSDate: Get interval from rhs object in comparison

* Add prefix to CONCRETE_CLASS_NAME  macro

Source/NSDatePrivate.h

@@ -0,0 +1,41 @@
+/** NSDate Private Interface
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   Written by: Hugo Melder <hugo@algoriddim.com>
+
+   This file is part of the GNUstep Base Library.
+
+   This library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2 of the License, or (at your option) any later version.
+
+   This library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with this library; if not, write to the Free
+   Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+   Boston, MA 02110 USA.
+*/
+
+#import <Foundation/NSDate.h>
+
+#if defined(OBJC_SMALL_OBJECT_SHIFT) && (OBJC_SMALL_OBJECT_SHIFT == 3)
+#define USE_SMALL_DATE 1
+#define DATE_CONCRETE_CLASS_NAME GSSmallDate
+#else
+#define USE_SMALL_DATE 0
+#define DATE_CONCRETE_CLASS_NAME NSGDate
+#endif
+
+@interface DATE_CONCRETE_CLASS_NAME : NSDate
+#if USE_SMALL_DATE == 0
+{
+@public
+  NSTimeInterval _seconds_since_ref;
+}
+#endif
+@end