add a comment about __OBJC__ define

Author: itrofimow

libcxx/include/__exception/exception_ptr.h

@@ -92,6 +92,7 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
 template <class _Ep>
 _LIBCPP_HIDE_FROM_ABI exception_ptr make_exception_ptr(_Ep __e) _NOEXCEPT {
 #  if _LIBCPP_HAS_EXCEPTIONS
+  // Clang treats throwing ObjC types differently, and we have to preserve original throw-ing behavior
 #    if !defined(__OBJC__) && _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION && __cplusplus >= 201103L
   using _Ep2 = __decay_t<_Ep>;