[clang-tools-extra] [clang-tidy] Add bugprone-custom-errno-declaration check (PR #213015)

[Csaba Zeitvogel via cfe-commits]

zeitcsabi wrote:

@EugeneZelenko @vbvictor @zeyi2 Thank you all for the helpful reviews! I think I have addressed all of your suggestions.
Sorry for the delayed responses, and thanks for your patience with my first PR :)
Ready for further feedback if anything else comes up!

https://github.com/llvm/llvm-project/pull/213015