libc++: avoid use of deprecated __reference_binds_to_temporary

This was removed in upstream libc++ in commit
437ad06f762ab07d89badecdd20627db200b98d3, but as this does not apply
cleanly to the current repository, I am applying the equivalent change
in a minimally invasive way. This is needed to build with latest clang
HEAD as of today.

Reviewed by: dim
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52530