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

[Balázs Benics via cfe-commits]

balazs-benics-sonarsource wrote:

> 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 😄 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.

Changed the debugging interface a bit, and added comments explaining it in f339eb1dc8216da013d5a92f01332b43e0a75790

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