Update libstdc++ hack comment

This libstc++ hack isn't ready for removal. Updating the comment to
note what I found. While I have not proven Ville's
__is_throw_swappable patch made this go away, that patch did remove
the use of noexcept(noexcept(swap(....))). I'm not sure when gcc grew
deferred noexcept parsing.

Differential Revision: https://reviews.llvm.org/D101441