Instead, simply track it as a valid binding pattern. To do this in the case of
parenthesized formals, we delay throwing the binding pattern error for
parenthesized (and async "calls") until we know it's not an arrow function head
by itself.

This guarantees that if an arrow head is a valid binding pattern, it's either a
valid parenthesized head or a valid identifier, or invalid pattern ("array" or
"object" literal style). We can detect the latter case by checking that the
current token is not a RPAREN and the expression isn't an identifier.
(Alternatively we could check that the curren token is RBRACE or RBRACK...)

Bug: chromium:907575
Change-Id: Ie40cc3235d3188f2620b6c089a0f49d93604dda6
Reviewed-on: https://chromium-review.googlesource.com/c/1348078Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57743}