[Chapel Merge] Use performance playground for LLVM 18

Branch: refs/heads/main
Revision: 9688573c09bc94e9159f209941ce26d2f06bace4
Author: jabraham17
Link: Use performance playground for LLVM 18 by jabraham17 · Pull Request #24893 · chapel-lang/chapel · GitHub
Log Message:
Use performance playground for LLVM 18 (#24893)

Test new LLVM 18 support in perf playground

Not reviewed - trivial

Compare: Comparing c5bc1fa172f934156c96624ed51fcfa751d1ed88...a5d3183b26909a337a5d606a275f13f9c01acf00 · chapel-lang/chapel · GitHub

Diff:
M util/cron/test-perf.chapcs.playground.bash
https://github.com/chapel-lang/chapel/pull/24893.diff