FreeBSD has made a lot of progress with clang in the base system (which consists of both the kernel and standard userland); I run a clang-compiled FreeBSD on my laptop.

But that said, we have a lot more clang coverage through the work done by the ports team who have been running experimental builds with clang for more than a year. That work is documented here: http://wiki.freebsd.org/PortsAndClang