Indeed. For example FreeBSD (the kernel and the ports) can now be built with Clang:
An unmodified FreeBSD head and stable/9 can completely be built by clang! (Note that you still need to setup /etc/make.conf or /etc/src.conf properly, see below for the details.) [1]
For 10-CURRENT after OSVERSION 1000024 (svnref 242624) Clang is the default compiler on i386 and amd64. And is installed as cc, c++ and cpp. [2]
An unmodified FreeBSD head and stable/9 can completely be built by clang! (Note that you still need to setup /etc/make.conf or /etc/src.conf properly, see below for the details.) [1]
For 10-CURRENT after OSVERSION 1000024 (svnref 242624) Clang is the default compiler on i386 and amd64. And is installed as cc, c++ and cpp. [2]
[1] https://wiki.freebsd.org/BuildingFreeBSDWithClang
[2] https://wiki.freebsd.org/PortsAndClang