Spec Text:
https://tc39.es/proposal-temporal/#sec-temporal.plainyearmonth.prototype.toplaindate
https://tc39.es/proposal-temporal/#sec-temporal.plainmonthday.prototype.toplaindate

Bug: v8:11544
Change-Id: I506efe3fa3685f621596fa34301a73d8c57ded38
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3566114
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80783}