Use PODArray<> in SparseSet.
Use PODArray<> in SparseSet.

Change-Id: Id5bf117f1dd68494e2f502add97e3c6331d206d6
Reviewed-on: https://code-review.googlesource.com/c/37290
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

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