AFAIK, LLVM development is highly moving on and it's API is changing.
So tools using LLVM (e.g. stack) require specific version of LLVM and building & installing required version of LLVM makes installation hard.
So I think bundling LLVM libraries to repository itself is one of the good choice.
LLVM allow us to build it as static library, so it's easy to bundle it for stack.
For example, mozilla/rust compiler bundles LLVM with modification for rust[1].
What do you think of this?