[clang-tools-extra] [clang-tidy] Add new check readability-trailing-comma (PR #173669)

[Victor Chernyakin via cfe-commits]

https://github.com/localspook commented:

I feel like the `Threshold` options may not be enough to capture the logic people use to decide whether they want a trailing comma or not. Anecdotally, I feel the rule is more "use a trailing comma iff the enum/init-list spans multiple lines" than "use a trailing comma if it has at least N elements".

https://github.com/llvm/llvm-project/pull/173669