llvm now split to different packages

- llvm, llvm-libs (in dir llvm/): the main llvm lib and functional part
- lld: (dynamic) linker of llvm
- lldb: debugger of llvm
- compiler-rt: runtime compiler lib for clang to build and run
- clang: C/C++ compiler from llvm project

they should stay in same release version
the update sequence is:
(optional ocaml group->) llvm -> lld -> compiler-rt -> clang