[libc++] Add _LIBCPP_NODEBUG to __unwrap_ref_decay_t

`__unwrap_ref_decay_t` should be marked `_LIBCPP_NODEBUG`, since it's
and internal type alias and this causes our CI to fail currently, since
we started enforcing this policy via clang-tidy.

GitOrigin-RevId: 3b92cf281a20a90717e481f80cc010da8096c249
1 file changed