Allow llvm16

[mbouaziz]

Part of my work towards #734

I probably won't go further for the time being.

Before merging, I want to:

• push docker images with LLVM 16 and skiplang built with it
• check all CI jobs are still green

[beauby]

Nice. If we start supporting several versions of LLVM, we should have CI jobs for the compiler tests using each supported (major) version.

[mbouaziz]

We probably don't want to have the burden of maintaining compatibility with several versions, either they are 99% compatible and we only have to worry when someone enters the 1% edge case, or we have to stick to one.

The CI's not happy so I'll probably give up until it becomes a higher priority issue

[jberdine]

The skipruntime test is flaky, rerunning it passed.

[bennostein]

The skipruntime test is flaky, rerunning it passed.

I suspect a race like the one Lucas hit -- working on some fixes to make them more robust to timing variability.

[mbouaziz]

Shall we merge this? (without the last commit) considering LLVM 15 and 16 seem compatible enough for our use cases.

Or do you think we should allow only one version?

[mbouaziz]

Closing in favor of what @beauby is currently doing