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

[Micah Weston via llvm-commits]

https://github.com/red1bluelost commented:

I see two parts in this PR: 
1) mapping the perf profile directly to the BBAddrMap graph
  * I could see this possibly being useful here after a redesign. Maybe emit a JSON, CSV, YAML, etc that contains block ID and relevant perf counts. Users could consume this as they see fit.
2) using that mapping to compute branch accuracy
  * I think this ends up being too user specific around how metrics are defined. There was similar discussion in my [RFC thread](https://discourse.llvm.org/t/rfc-pgo-accuracy-metrics-emitting-and-evaluating-branch-and-block-analysis/73902/1) which was why I did not try to upstream the metrics I had.

I am no longer actively working in the PGO area so I would defer to others on whether these features are desired in upstream. I'm happy to review code if these features move forward.

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