[llvm] [CMake] Add root CMakeLists.txt to allow direct CMake execution from root (PR #152753)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

Doing it like this means that all the build products end up in e.g. `llvm/bin` and `llvm/lib` relative to the current build directory - while we'd want to keep them in `bin` and `lib` directly at the top level of the build directory. Keeping that aspect while building from the top directory will take a bit more work.

https://github.com/llvm/llvm-project/pull/152753