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

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

Very nice. Thanks!

> Documentation has been moved from .cpp files to the header. Grammar was consistently put in \verbatim blocks to render nicely in Doxygen.

Especially this part; this has bothered me in the past. ;Þ

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