Update LLVM trunk version to 16

[gonzalobg]

Pull Request Description

Author Checklist

• [x] Updated documentation (pydocmd generate) if any docstrings have been modified
• [x] Passes all unit tests

[gonzalobg]

@samcmill could you do a release after merging this one? Our workflow depends on this and having to fall back to LLVM 14 is not great.