libc++ backport to stretch

[Emanuele Rocca]

Recent versions of clang such as 5 and 6 are available in
stretch-backports. However, there's no corresponding package providing
an up-to-date version of the C++ standard library. For this reason,
support for recent ISO C++ standards such as C++17 is currently
incomplete in stretch-backports.

I've discussed this on #debian-llvm and the consensus was that it does
make sense to upload libc++ 6.0.1-1 to stretch-backports (no changes
required): https://salsa.debian.org/pkg-llvm-team/libcxx/merge_requests/1

Any objections?