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

[via llvm-commits]

lifengxiang1025 wrote:

> 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.
> 
Yes. I'm not thinking carefully about the output format. However, maybe there is no perfect method to define output. Sometimes I think `llvm-profdata overlap`'s output is not good for me. But its output can help me know the general result. I think we can upstream the most general output firstly and upstream more flexible output later.

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