Add UNSUPPORTED annotation

Author: ldionne

libcxx/test/libcxx/vendor/apple/disable-availability.sh.cpp

@@ -8,6 +8,10 @@
 
 // REQUIRES: stdlib=apple-libc++
 
+// This test is dependent on the code generated by the compiler, and it doesn't
+// work properly with older AppleClangs.
+// UNSUPPORTED: apple-clang-15
+
 // This test ensures that we retain a way to disable availability markup on Apple platforms
 // in order to work around Clang bug https://github.com/llvm/llvm-project/issues/134151.
 //