[clang-tools-extra] [clang-tidy] Refactor the build system (PR #173518)

[Baranov Victor via cfe-commits]

vbvictor wrote:

What about downstream users that has their own modules with checks? There are major public users like mozilla, libc++ and countless private companies.
I am worried that could bring a lot of breakage and churn downstream.

Also, we shouldn't merge any major now because downstream folks wouldn't have time to test property because of Christmas/New year 



https://github.com/llvm/llvm-project/pull/173518