The `PerformPromiseThen` spec-internal operation is used by the async functions
proposal, in order to ensure that AwaitExpressions are not observable via
usual mechanisms/hooks, such as Symbol.species.

BUG=v8:5253
R=littledan@chromium.org, adamk@chromium.org, gsathya@chromium.org, yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2209433003
Cr-Commit-Position: refs/heads/master@{#38353}