Track whether the async arrow func parameter list was simple or not; the
information is already there, we just didn't pipe it through correctly. It's
needed by PreParser so that it can create the correct Scope structure.

Implementation notes:

- I could've used async_classifier for transmitting the "is_simple" bit, but I
  made it explicit (it would be unnecessary to use ExpressionClassifier for
  this, as we're not classifying any expressions) instead.

- I'm also moving work (setting parameter_list.is_simple) from Parser to
  ParserBase, and adding a DCHECK in Parser to assert that the work was indeed
  already done.

BUG=v8:5516,chromium:765532

Change-Id: Iacf91b150d1b57996544b5e64baa7d91ac134445
Reviewed-on: https://chromium-review.googlesource.com/674695
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48132}