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

[Owen Pan via cfe-commits]

owenca wrote:

This patch makes `isProto()` a convenience function like `isCpp()`, which doesn't prevent us from handling C++ and Objective-C separately when needed. Although `LK_TextProto` is not always formatted like `LK_Proto`, it's frequent enough (about 16 times in the codebase) that we should make a function for it.

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