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

Following the steps of #82217, this patch reorganizes declarations in
`Parse.h`. Highlights are:
1) Declarations are grouped in the same fashion as in `Sema.h`. Table of
contents is provided at the beginning of `Parser` class. `public`
declaration go first, then `private` ones, but unlike `Sema`, most of
the stuff in `Parser` is private.
2) Documentation has been moved from `.cpp` files to the header. Grammar
was consistently put in `\verbatim` blocks to render nicely in Doxygen.
3) File has been formatted with clang-format, except for the grammar,
because clang-format butchers it.

0 files changed, 0 insertions, 0 deletions