[libcxx] Fix buildbots on Debian

Debian build bots are running Clang 4, which apparently does not support
the "deprecated" attribute properly. Clang pretends to support the attribute,
but the attribute doesn't do anything.

(live example: https://wandbox.org/permlink/0De69aXns0t1D59r)

On a separate note, I'm not sure I understand why we're even running the
libc++ tests under Clang-4. Is this a configuration we support? I can
understand that libc++ should _build_ with Clang 4, but it's not clear
to me that new libc++ headers should be usable under older compilers
like that.

llvm-svn: 342854