Simplify StateSet and DFA::ClearCache().
Simplify StateSet and DFA::ClearCache().

Change-Id: Ifc1167504284189d203a2e84f667ba38070ff6ec
Reviewed-on: https://code-review.googlesource.com/5254
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2