We used to point elsewhere, for instance the right-hand-side of an assignment.

Small limitation: Since variable proxies only have a start position, not an end
position, the best we can do is point at the first character.  (We cannot rely
on the scanner's last token position because Declare may be called long after
the variable has been scanned.)

R=adamk@chromium.org
BUG=v8:5572

Review-Url: https://codereview.chromium.org/2447143005
Cr-Commit-Position: refs/heads/master@{#40613}