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

[Daniil Dudkin via cfe-commits]

unterumarmung wrote:

@EugeneZelenko, thank you for your thorough reviews of the docs. Could you point me to a document that outlines all the formatting rules? I'd really appreciate having access to it so I can address any issues more independently in the future.

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