Currently, PRIVATE_NAME / PrivateIdentifier is not valid in
ObjectLiterals or other places expecting the PropertyName production.
A SyntaxError here prevents an access violation later on when attempting
to dereference a null property key

BUG=v8:8808
R=gsathya@chromium.org, littledan@chromium.org

Change-Id: Idde9c669cb48c1595b83115351a8fe0caed40eef
Reviewed-on: https://chromium-review.googlesource.com/c/1461161Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#59515}