Since contrib/libc++'s ancestry was never correct, subversion 1.8 and

Since contrib/libc++'s ancestry was never correct, subversion 1.8 and
higher cannot merge to it from the vendor area.

Re-bootstrap the ancestry by doing (apologies to your retinas):
* svn rm contrib/libc++
* svn cp ^/vendor/libc++/dist@276792 contrib/libc++
* svn cp ^/head/contrib/libc++/FREEBSD-upgrade@287629 contrib/libc++
* svn cp ^/head/contrib/libc++/include/__bit_reference@287629 contrib/libc++/include
* svn cp ^/head/contrib/libc++/include/__config@287629 contrib/libc++/include
* svn cp ^/head/contrib/libc++/include/__tree@287629 contrib/libc++/include
* svn cp ^/head/contrib/libc++/include/algorithm@287629 contrib/libc++/include
* svn cp ^/head/contrib/libc++/include/stdexcept@287629 contrib/libc++/include
* svn cp ^/head/contrib/libc++/include/type_traits@287629 contrib/libc++/include
* svn pd -R svn:mergeinfo contrib/libc++
* svn ps -F mergeinfo.txt contrib/libc++