How to update LLVM & Clang on Mac OSX?

The web site is incorrect. Xcode 4.2.1 and 4.2 include LLVM 3.0 and clang 3.0:

clang --version
Apple clang version 3.0 (tags/Apple/clang-211.12) (based on LLVM 3.0svn)

clang++ --version
Apple clang version 3.0 (tags/Apple/clang-211.12) (based on LLVM 3.0svn)

llvm-g++ --version
i686-apple-darwin11-llvm-g++-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00)

llvm-gcc --version
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00)

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.