MFC r260015:

MFC r260015:

In libc++'s type_traits header, avoid warnings (activated by our use of
-Wsystem-headers) about potential keyword compatibility problems, by
adding a __libcpp prefix to the applicable identifiers.

Upstream is still debating about this, but we need it now, to be able to
import clang 3.4.

git-svn-id: svn://svn.freebsd.org/base/stable/10@260263 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f