[llvm] [gardening][DebugInfo][NFC] Improve comment on HashingDISubprogram test (PR #91543)

[David Blaikie via llvm-commits]

dwblaikie wrote:

Oh, and also, for now (though there's some discussion about changing this) you don't need to send pull requests for commits that you aren't seeking precommit approval for. Some folks send PRs in that case just to run the automated precommit testing, but presumably you can be confident enough in a comment-only change that rerunning precommit testing on it isn't especially valuable (:

https://github.com/llvm/llvm-project/pull/91543