Await is a unary operator and should be disallowed on the LHS of
exponentiation like all other unary operators.

Bug: v8:11213
Change-Id: I9c51e33cb37660627748cd926ec222ac0ac246de
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2566442Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71535}