[clang-tools-extra] [clang-tidy] Add `readability-redundant-nested-if` check (PR #181558)

[Daniil Dudkin via cfe-commits]

unterumarmung wrote:

@vbvictor I've tried to address most of your comments. Maybe I forgot something. I also simplified the check on `if constexpr` handling. 

https://github.com/llvm/llvm-project/pull/181558