Simpliy track arrow-formal-parameters through errors and copy it from binding
pattern errors in AccumulateImpl rather than using a separate flag.

Change-Id: I8146718f490f3c9a5de3c30dec2aa3218831523f
Reviewed-on: https://chromium-review.googlesource.com/c/1275816
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56568}