Another (IMHO better) validation is that FreeBSD compiles the kernel with clang: http://wiki.freebsd.org/BuildingFreeBSDWithClang

FreeBSD can compile the kernel with Clang, but at least as of 9.0-RELEASE, the default is still GCC. They're aiming to switch to Clang for as much as possible for 10.0-RELEASE, in a couple of years.

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

Isn't most of their code in Objective C? Does that use clang too?

clang does support Objective C (as does gcc).

Apple does look after a lot of plain old C (e.g. Core Foundation) and C++ (e.g. WebKit) too, so clang handles all three.

If I'm not mistaken, the drivers are also written in C++ (They used to be in Objective-C).