Use the position of commas in arrow expressions to mark the initializer
position of any parameters that might have been set in the preceding
parameter.

To enable this, this makes variable_list_ in ExpressionParsingScope a
ScopedList<pair<VariableProxy*, int>> and changes ScopedList::at to
return references so its elements can be modified in place.

This fixes a source of bytecode mismatches when collecting source
positions lazily and is a second attempt at fixing this after
https://chromium-review.googlesource.com/c/v8/v8/+/1683267 introduced
problems due to destructuring.

Bug: chromium:980422, chromium:981701, v8:8510
Change-Id: I948f89f34fb75d7463a13183e363f7f96ad09d13
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1710671Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62936}