[llvm] [NFC] [DebugCounter] warn if --debug-counter is unused in NDEBUG (PR #143057)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM

I was wondering whether we could just unconditionally enable debug counters, but doing that does have some meaningful overhead: https://llvm-compile-time-tracker.com/compare.php?from=100a1d0c4caad0d0f2ec26b07d3cc73f59b9a9a8&to=341c03268229f2982df1fd7ddb5e3db7a2e7731a&stat=instructions:u

https://github.com/llvm/llvm-project/pull/143057