This deletes unresolved VariableProxy objects created for labels in the
preparser which prevents shadowed variables in enclosing scopes from
being context-allocated.

Previously this was only done in the full parser, which leads to
bytecode mismatches with lazy source positions.

Bug: chromium:1009728, v8:8510
Change-Id: If2d0c345346116a7f5aacbcd0cf3638e9f7e04cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1836258Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64104}