Fix type-punned dereference warning

[BsAtHome]

This fixes #3204.

No formats or sizes change. Just the way the value of the enum is addressed. The enum was already defined as an enum class which has int as the default underlying type. This has not changed but the underlying type is made explicit because the union absolutely needs both fields as ints to match size (and alignment). See also added code comments.

[andypugh]

Thanks for your work on this. (and everything else)