Clang 3.7 Plugin for Xcode
brew install llvm --with-clang --with-lld --HEAD
If your homebrew is setup to install to /usr/local after a lengthy build process you will end up with the most up to date LLVM available installed at /usr/local/opt/llvm.
If your homebrew is setup to install somewhere else, you will need to edit the file Clang 3.7.xcplugin/Contents/Resources/Clang 3.7.xcspec in Xcode. What you want to change is Item0.ExecPath to the full path to your brew installed clang executable (defaults to /usr/local/opt/llvm/bin/clang), Item1.ExecCplusPlusLinkerPath to the full path to your clang++ executable, and Item2.ExecPath to the full path to your clang as in Item0.ExecPath).
Copy the Clang 3.7.xcplugin directory into /Applications/Xcode.app/Contents/PlugIns/Xcode3Core.ideplugin/Contents/SharedSupport/Developer/Library/Xcode/Plug-ins.
Restart Xcode. You should be able to select LLVM 3.7 under Build Options -> Compiler for C/C++/Objective-C.