Use LLVM Snapshot in CI

[hmelder]

https://apt.llvm.org/ provides a development snapshot. It might be good to have one CI run using this snapshot build.

[davidchisnall]

It’s probably worth running that as a nightly or weekly job, to see if any LLVM changes broke things. That hasn’t happened for a few years, but it remains possible.

[hmelder]

Should we run this on x86 only or also on the cross builds?

[davidchisnall]

The cross builds probably don't add very much extra overhead.