When parsing a pattern element with an assignment operator that is not
Token::ASSIGN, record a pattern error to indicate the invalid assignment target.

BUG=v8:811, v8:4666
LOG=N
R=adamk@chromium.org, rossberg@chromium.org, littledan@chromium.org

Review URL: https://codereview.chromium.org/1583863003

Cr-Commit-Position: refs/heads/master@{#33279}