Keep Coding

The highest version of Xcode that is officially supported on OS X 10.8.5 is Xcode 5.1.1, and that comes with Apple LLVM 5.1 (based on LLVM 3.3svn). However, while newer versions of Xcode cannot be installed on Mountain Lion, the command-line tools that are bundled with Xcode work perfectly fine. They can be extracted from the package installer using an app like Pacifist.
So now I can compile using Apple LLVM 6.0 (based on LLVM 3.5svn) from the command line, but does anyone know if it's safe to replace the version of clang Xcode uses with this newer version?

not at all - no
you have NO way of knowing what apple changed / compiled into that version
it IS safe to replace clang/llvm, but not with a version you know nothing about