[clang-tools-extra] [clang-tidy][NFC] Don't store a redundant copy of `HeaderFileExtensions` in every check (PR #190302)

[Zeyi Xu via cfe-commits]

https://github.com/zeyi2 approved this pull request.

LGTM apart from open comments.

https://github.com/llvm/llvm-project/pull/190302