[clang-tools-extra] [clang-doc][NFC] refactor out file helpers (PR #134298)

[Paul Kirth via cfe-commits]

================
@@ -0,0 +1,72 @@
+//===-- FileHelpersClangDoc.cpp - File Helpers -------------------*- C++-*-===//
----------------
ilovepi wrote:

Apparently, we don't use that line anymore. https://llvm.org/docs/CodingStandards.html#file-headers

Was brought up here when I saw this the first time: https://github.com/llvm/llvm-project/pull/111487#discussion_r2028191191

https://github.com/llvm/llvm-project/pull/134298