Rollup merge of #122178 - cuviper:ci-llvm-18, r=Kobzol

ci: add a runner for vanilla LLVM 18

For CI cost, this can be seen as replacing the llvm-15 runner we dropped in #117947.

Also, I've set `IS_NOT_LATEST_LLVM` in the llvm-17 runner, since that's not the latest anymore.

Author: matthiaskrgr