Auto merge of #116950 - cuviper:ci-llvm-17, r=Mark-Simulacrum

ci: add a runner for vanilla LLVM 17

For CI cost, this can be seen as replacing the llvm-14 runner we dropped in #114148.

Also, I've set `IS_NOT_LATEST_LLVM` in the llvm-16 runner, since that's not the latest anymore.

Author: bors