This reverts commit f2e25f8a.

Reason for revert: Breaks GCC 8.4.0 compilation, see bug.

Original change's description:
> [base] Replace base::Optional with Abseil
>
> Change base::Optional to an alias of absl::optional. Eventually we
> should remove it entirely.
>
> Bug: v8:11006
> Change-Id: I687d44cc7e7cd0a49a84bcc207231eb6808eef2d
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2476318
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#76913}

Bug: v8:11006, v8:12252
Change-Id: I8048dee676a36551d15f25a31d1e171dae00d975
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3173672
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#76976}