With the latest (appstore) XCode 4.3.2, the location changed, it can now be found in
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/libclang.dylib
The /Developer directory, among others, no longer exists by default. Everything is now packaged inside the XCode application, so that delta updates from the appstore work.