[llvm] [llvm-profgen] Add pgo accuracy metrics (PR #93975)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

I'd recommend posting a RFC on discourse [referencing @red1bluelost's](https://discourse.llvm.org/t/rfc-pgo-accuracy-metrics-emitting-and-evaluating-branch-and-block-analysis/73902), clarifying in particular why this needs to be in the tree as opposed to something in out-of-tree tools.

To be clear, I'm not pushing back. I think, though, that this is worth a bit more discussion in the community.

https://github.com/llvm/llvm-project/pull/93975