On my version of Mac OSX (Lion 10.7.1, XCode 4.1), there is LLVM 3.0svn and Clang 2.1. The current versions are LLVM 3.0 and Clang 3.0.
From the XCode web site, it s
Software vendors are traditionally conservative about updating build tools and with good reason. If you want to use the latest publicly-supported versions of build tools supplied and modified by Apple, you should stick to those in the latest version of Xcode for the OS X release you are running. There are usually good reasons why Apple has not yet updated to the latest cutting-edge versions of open source components, like serious bugs. If you don't need the Apple-supplied modifications and don't mind living on the edge - i.e. no support from Apple and possibly (re-)discovering known problems - and are not planning to ship compiled files to other people's systems, you could install your own versions in, say, /usr/local/bin or by using third-party package managers, like MacPorts http://www.macports.org/ports.php?by=name&substr=clang). You should definitely not try to replace the files at the paths installed by Xcode. Is it worth it? Only you can decide that.