Add experimental support for unofficial monorepo-like directory layout

Summary:
This allows to have clang and llvm and the other subprojects
side-by-side instead of nested. This can be used with the monorepo or
multiple repos.

It will help having a single set of sources checked out but allows to
have a build directory with llvm and another one with llvm+clang.
Basically it abstracts LLVM_EXTERNAL_xxxx_SOURCE_DIR making it more
convenient by adopting a convention.

Reviewers: bogner, beanz, jlebar

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D26365

llvm-svn: 286162