Add LLVM project as a submodule.

[icmccorm]

Adds llvm-project as a submodule.

This is a more sustainable configuration than relying on the contents of our sysroot, since that will be subject to change if our retags are eventually accepted in nightly.

./xb setup will perform a sparse checkout with the contents of llvm-sparse. All changes are ignored; we should never fork LLVM. It's only present as a dependency for our LLVM wrapper.