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

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat approved this pull request.

I also read the tests and the change LGTM if you move the explanations from the very helpful review comments https://github.com/llvm/llvm-project/pull/140924/files#r2107471659 and https://github.com/llvm/llvm-project/pull/140924/files#r2107491565 to source code comments.

In the tests I felt that it'd be a bit hard to decipher the meaning of the block identifiers like `B1` etc. -- but when I re-read the file I noticed that you included the very nice helper function `dumpCFGAndEgraph` (IIUC) for those who will need to debug broken cases in this test file :smile: Perhaps it would be even nicer if you included a commented out call to that function with "`// NOTE: Uncomment this if you want to decipher the meaning of 'B0', 'B1', ...`" to make its existence and role more obvious.

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