Summary:
This allows using the upsteam LLVM 11 library unchanged, only
extensions to the OCaml bindings are needed. Therefore this is to
enable building sledge using e.g. `dnf install llvm-11` or `brew
install llvm@11` instead of cloning and building a fork of llvm.
Reviewed By: jvillard
Differential Revision: D27188301
fbshipit-source-id: f441dbecd