[clang-tools-extra] [clang-tidy] Add avoid-pragma-once. (PR #140388)

[Baranov Victor via cfe-commits]

https://github.com/vbvictor commented:

Some more nit changes.
If you will to contribute further, reading https://clang.llvm.org/extra/clang-tidy/Contributing.html may help you.
Particularly "Making your check robust" and "Documenting your check". Although, most of it already covered in comments. 

https://github.com/llvm/llvm-project/pull/140388