[llvm] [nfc] Improve testability of PGOInstrumentationGen (PR #104490)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

* **#104491** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/104491?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a>
* **#104490** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/104490?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a> 👈
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https://github.com/llvm/llvm-project/pull/104490