[Frontend] Introduce `getDirectiveCategory` for ACC/OMP directives (#94689)

The categories are primarily meant for OpenMP, where the spec assigns a
category to each directive. It's one of declarative, executable,
informational, meta, subsidiary, and utility.

These will be used in clang to avoid listing directives belonging to
certain categories by hand.

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Co-authored-by: Valentin Clement (バレンタイン クレメン) <clementval@gmail.com>