Note that even if the build succeeds, many Julia packages probably won’t work well, and they will refuse to accommodate USE_SYSTEM_LLVM=1 should you find and report errors.
I say this from the side of a package developer not interested in supporting it (although I’d gladly accept PRs fixing problems).
I say this to discourage you, unless it is something you really want and are willing to put in the work for.
If all you want is a newer LLVM, checkout this PR.