[clang-tools-extra] [clang-tidy][NFC] add '.clang-tidy' config for clang-tidy project (PR #147793)

[via cfe-commits]

EugeneZelenko wrote:

How about adding `modernize` relevant to currently used C++ standard? Also some of `readability` are useful. Please also consider top-level `.clang-tidy` file re-formatting.

https://github.com/llvm/llvm-project/pull/147793