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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> I think downstreams need some kind of notification that this large of a change is coming; can you post something to Discourse to alert them and encourage them to mention any major hardships on this PR?

https://discourse.llvm.org/t/parser-h-reorganization-pr/86178

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