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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM! Assuming we don't get push-back by Friday from downstreams on the Discourse post or here on the PR, I think it's fine to land. (I think downstreams need more than ~2 days to determine impacts, but ~5 days is a bit more reasonable.)

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