Previously, b6e9f625 fixed self-assignment
in parameters to throw. But it failed to deal with the case of
destructuring with defaults. This patch extends that previous approach
to always treat the end of a parameter as its initializer position,
whether it has an initializer or not.

This is the minimal change to make it easy to merge; a follow-up
will rename the field of Parameter from "initializer_end_position"
to "end_position".

BUG=v8:5454

Review-Url: https://codereview.chromium.org/2390943002
Cr-Commit-Position: refs/heads/master@{#39962}