Delay the creation of FunctionNameVariables until we validated the
FormalParameters. This is needed so we don't declare them in cases where
we later get an error, have to reset, and reparse.

Bug: chromium:890553, v8:7926
Change-Id: I742e6f7f71158e3903843bd583dc7943468c18f6
Reviewed-on: https://chromium-review.googlesource.com/1254061Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Florian Sattler <sattlerf@google.com>
Cr-Commit-Position: refs/heads/master@{#56314}