If you want to contribute the best thing to do is to build everything from source. It takes a while, though, be prepared.
Warning: WORK IN PROGRESS
This is how I've been doing it so far, which is far from ideal, and only on Mac
Build LLVM. Warning: this might take hours.
Download the LIL sources from GitHub
Create a new Xcode project. Choose Command Line Tool as the project type.
Add all source files to the project
Add all folders as "Search Header Paths"
Add the LLVM static libraries to link to them
Hit build
(Optional) Configure arguments to be passed at launch so you can use Build & Run. This is done by editing the scheme in Xcode: Product -> Scheme -> Edit Scheme...