re2: use std::optional instead of absl::optional

std::optional is now preferred over absl::optional, even at Google.
It was introduced in C++17.

Change-Id: If02413a46299beef521effaad265ca04f43088fa
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63771
Reviewed-by: Alan Donovan <adonovan@google.com>