Only support LLVM 16

[dpaoliello]

As discussed in #60 we will only be supporting one version of LLVM at a time. I've already created a tag for LLVM 10: https://github.com/JuliaHubOSS/llvm-cbe/tags

• Enforce LLVM 16 in CMake.
• Update README.
• Remove #if for different LLVM versions.
• Other minor cleanups.