[clang] [analyzer] Introduce the check::BlockEntrance checker callback (PR #140924)

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

> What is the relationship of this new callback with the BranchCondition callback

+1, I am also interested to learn what is the main motivation behind the new callback. Do you have some example use cases in mind? I think users might get confused which one to use. 

https://github.com/llvm/llvm-project/pull/140924