Spec Text: https://tc39.es/proposal-temporal/#sec-temporal.plaintime.prototype.toplaindatetime

Bug: v8:11544
Change-Id: I95bab9814471bb9347101d654f6dc902159f8fe3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3538670Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80757}