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

[David Blaikie via llvm-commits]

dwblaikie wrote:

Thanks for the comment update!

(though if you send something for review, please wait for the review to be completed before submitting - if you didn't intend this PR to be for precommit review, please use the `skip-precommit-approval` label on the PR ( https://llvm.org/docs/GitHub.html#pull-requests ))

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