Rename to Regexp::RequiredPrefixForAccel().

Change-Id: I72084774560d1989d5eac937f4a5039802713d69
Reviewed-on: https://code-review.googlesource.com/c/re2/+/56772
Reviewed-by: Paul Wankadia <junyer@google.com>
(cherry picked from commit 25238e4a33af97b16156f56b4fe7bd7a4e46b182)
Reviewed-on: https://code-review.googlesource.com/c/re2/+/56773
4 files changed