The llvm-monorepo-root, llvm-project-v4, llvm-project-v4-split, llvm-project-v3, llvm-project-v3-split, llvm-project-v2, and llvm-project-v2-split repositories are WIP repositories that are used for development and prototyping of an llvm-project monorepo that will be used for Apple's compiler releases and for the open source Swift project. Please see this forum post for more details.
Original readme follows:
This directory and its subdirectories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments.