[clang] [clang-format][NFC] Extend isProto() to also cover LK_TextProto (PR #73582)

[Björn Schäpers via cfe-commits]

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

I must've read some of the diff backwards. I thought you replaced `Language == LK_Proto` with `isProto()`.

https://github.com/llvm/llvm-project/pull/73582