[clang] Implement KeepEmptyLines.AtEndOfBlock feature (PR #224223)

[Arkadiusz Bokowy via cfe-commits]

arkq wrote:

> * [LLVM AI Tool Use Policy](https://llvm.org/docs/AIToolPolicy.html)

I've read the policies and no AI has been used when working on the PR.

https://github.com/llvm/llvm-project/pull/224223