Get rid of `using std::string;`. (part 2 of N)

Change-Id: I3451b06b238ad93eefc1e2b387351ec6f0ff792d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/39112
Reviewed-by: Paul Wankadia <junyer@google.com>
6 files changed