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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

Numbers for clang-format uncleanliness of `Parser.h` with changes from this PR: `92 insertions(+), 87 deletions(-)` (all in grammar productions).

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