Added Clang to repository.

The old system (checkout out Clang in scripts/init) obviously couldn't
work, due to version drift. Thus, this versioned reference.

I'm using "obviously" in a very loose sense here, since obviously it
wasn't so obvious to me back when I set this up. Oh well.
Upgraded LLVM to latest version.

This version matches the Clang one.
Updated scripts.

They now work with the new repository structure (clang being included as
a submodule).