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

[Björn Schäpers via cfe-commits]

https://github.com/HazardyKnusperkeks commented:

I can not say if TextProto should be handled equally to Proto.

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