[llvm] [CI][format] Explicitly pass extensions to git-clang-format (take 2) (PR #98227)

[Louis Dionne via llvm-commits]

ldionne wrote:

OK, sorry for the multiple updates here but I think this should be good to go. I tested with:

- a single `.cpp` file from LLVM
- a `.cpp` file from LLVM and an extension-less header from libc++
- a single extension-less header from libc++

and they all seemed to work now.

https://github.com/llvm/llvm-project/pull/98227