[LSan][Darwin][NFC] Add comment explaining test failure
// REQUIRES: stable-runtime
// XFAIL: ubsan
-// FIXME(dliew): Make this test work on Darwin with LSan
+// FIXME: On Darwin, LSAn detects the leak, but does not invoke the death_callback.
// XFAIL: darwin && lsan
#include <sanitizer/common_interface_defs.h>
