[llvm] [PGO] Drive profile validator from opt (PR #147418)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

> I'm assuming you've already looked at trying to just make `lit` add the flag to opt invocations rather than going through an additional CMake flag?

Yes. Main trouble is that I'd have to pass the full path to opt, besides the extra arg. What I did here is (subjectively) simpler.

https://github.com/llvm/llvm-project/pull/147418