[RFC] __attribute__((internal_linkage))

Thanks for reviving this!

Just as a clarification (seems relevant as I'm rereading this), I'm not
suggesting that we add _LIBCPP_INTERNAL_VISIBILITY to functions that are
already in the ABI, such as std::vector::__push_back_slow_path. Only to
change the internal-API function currently using _LIBCPP_INLINE_VISIBILITY


> The thread died by itself after this proposal.
> Should we move forward with this?

I'm in favour.