The process described on wiki.freebsd.org/BuildingFreeBSDWithClang seemed to be going too slowly, or completely stalled. However, a message from Roman Divacky recently appeared on the list that ClangBSD successfully builds itself on tier 1 platforms (i386 and amd64).
In short, ClangBSD is an attempt to compile the kernel and world of FreeBSD using the clang compiler, which, in turn, is a front-end to LLVM. As you know, FreeBSD still uses GCC version 4.2.1, since subsequent versions have changed the license. Clang also has a BSD license, and also has many other goodies. The project had problems with building libstdc ++, but now they have been overcome. The only known bug - the OS loader exceeds the size of the executable file. So far, there is a workaround - build a boot loader without UFS1 support.