[clang] [llvm] [Coverage] Introduce "partial fold" on BranchRegion (PR #112694)

[via cfe-commits]

Lambdaris wrote:

This patch also helps to distinguish constants from rust, in which constant may have only one counter hard coded to `Zero`, thank you!

Besides, I found that with this change we should also count false counter id when [calculating max `CounterID`](https://github.com/llvm/llvm-project/blob/eb9af19fbf13b91c9df8d6dc8553277169976520/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp#L610). Otherwise some cases would break with "unexpected" (actually expected!) counter id from profile data. I does so at [#94137](https://github.com/llvm/llvm-project/pull/94137/commits/755d8f253fc23b1ef1001095e0310d5f0fac07ab) and it would be also great if some one picked it and opened a smaller pr.

https://github.com/llvm/llvm-project/pull/112694