[llvm] format root clang-tidy config (NFC) (PR #147902)

[Baranov Victor via llvm-commits]

vbvictor wrote:

> LGTM. Worth noting is that the YAML list format is only supported since a few releases back, so it can break for users with older clang-tidy.

Thank you for noticing this, YAML list format is only 2 years old and LLVM promises backwards compatibility for 3 years of compilers. However, in CI we use `clang-format-20.1.5` so a user who formats code with `clang-format-19` could receive an invalid formatting error due to some bug that was fixed in `clang-format-20`.

Do we wish to support older versions of clang-tidy or just the latest one, WDYT?

My opinion is that as LLVM developers, we should use the latest version of our tools (that will run in CI), so having a config with new format would not be a big deal. People who use LLVM as a library and don't run clang-tidy over LLVM code, or do they?  

https://github.com/llvm/llvm-project/pull/147902