re2: use C++17 [[fallthrough]] instead of ABSL_FALLTHROUGH_INTENDED

According to [1], Google code now depends on C++17,
so we can use these directly.

[1] https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md

Change-Id: Ic475b79bc2153861d6ec2aebc0c55dbcf0c35908
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63770
Reviewed-by: Alan Donovan <adonovan@google.com>