As announced in mid April, RE2 will take a dependency on Abseil in the
`main` branch. This change is one of (quite probably) many that should
ultimately see the `abseil` branch removed.
Change-Id: I0ae5a6643f77d2ebf675443cea7cba558b8c7fef
Reviewed-on: https://code-review.googlesource.com/c/re2/+/61230
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
