Fixes a bytecode mismatch for arrow functions with default arguments
between eager and lazy compilation. In the former case, parameters with
default values are marked as assigned even if the value never changes
within the function because the parser does not know it's an
arrow-function at the point it sees the assignment.

So this changes ArrowHeadParsingScope::ValidateAndCreateScope to clear
the is_assigned flag on its parameter VariableProxies before it binds
them.

Bug: chromium:988304, v8:8510
Change-Id: I68bf205c73471386181e5fdcec6c8c3b2e527c8f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1724384Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62962}