[clang-tools-extra] [clang-tidy] Added check 'bugprone-function-visibility-change' (PR #140086)

[Piotr Zegar via cfe-commits]

https://github.com/PiotrZSL requested changes to this pull request.

Idea is ok.

Missing some tests (scenarios not covered).
Maybe some options to deal with big number of false-positives would be needed.
Maybe some option to allow changing visibility to less visible would be needed.

https://github.com/llvm/llvm-project/pull/140086