[clang] [clang][NFC] Regroup declarations in `Parser` (PR #138511)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> 3\. File has been formatted with clang-format, except for the grammar, because clang-format butchers it.

Yeah, this problem comes up rather frequently, I wonder if clang-format can get some additional smarts to make that less of a challenge? CC @mydeveloperday @owenca

https://github.com/llvm/llvm-project/pull/138511