[clang] [NFC] Optimize file kind determination (PR #139492)

[Juan Manuel Martinez Caamaño via cfe-commits]

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

Thanks @spavloff ! I'm sorry I didn't address the remarks earlier, I was out-of-office and didn't see the notifications.

https://github.com/llvm/llvm-project/pull/139492