[parser] Replace bool is_async/is_generator with ParseFunctionFlags

Change-Id: Idb0aa9a7ad246def358beae0b2b464496223d749
Reviewed-on: https://chromium-review.googlesource.com/1236575Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56100}