Cast to the underlying type for union tags

Union tags are of enum type. Streaming it would make more sense by
casting to underlying types.

For now casting is not required since tags are defined as `enum Tag`.
But we're going to change it to `enum class Tag` which won't work with
operator<< without casting.

Bug: 218912230
Test: m
Change-Id: Ia5e8a5c38fe23c72dffbdca320a32abdfa0eb38e