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

[Vlad Serebrennikov via cfe-commits]

Endilll 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

To be more specific, generally clang-format leaves grammar alone, but when it exceeds column limit, the result looks unacceptable for me.

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