Add some facilities to work with a git monorepo (experimental setup)

Summary:
Some changes are made to cmake, especially the addition of a new
LLVM_ENABLE_PROJECTS option that makes the build system aware of
the monorepo directory structure.

Also a new script is added in llvm/utils/git-svn/. When present in
the $PATH, it enables a `git llvm` command. It is providing at this
point only the ability to push from the git monorepo: `git llvm push`.
It is intended to evolves with more features, for instance I plan on
features like `git llvm show r284955` to help working with sequential
revision numbers.
The push feature is taken from Justin Lebar's script available here:
https://github.com/jlebar/llvm-repo-tools/

Reviewers: jlebar

Subscribers: mgorny, modocache, llvm-commits

Differential Revision: https://reviews.llvm.org/D26334

llvm-svn: 286123