[clang-tools-extra] [clang-tidy][NFC] Enable `readability-convert-member-functions-to-static` in the codebase (PR #156265)

[Victor Chernyakin via cfe-commits]

localspook wrote:

Tell me if I’m understanding correctly:

Your concern is with the API of a class. That means you’re concerned about changes to `public` and `protected` methods, but okay with changes to `private` ones?

https://github.com/llvm/llvm-project/pull/156265