DDSXTY14 — Representing union case labels in TypeObject

TypeObject assumes that all union case labels are representable in a 32-bit integer:

// Case labels that apply to a member of a union type
// Ordered by their values
typedef sequence<long> UnionCaseLabelSeq;

Both IDL 4.2 and the general type system in XTypes (Figure 18, 7.2.2.4.4.3) allow 64-bit integer types as discriminators and therefore 64-bit values as case labels.

Add support for unions with 64-bit discriminant to TypeObject

Add new Type Kind TK_UNION64 with associated CompleteUnion64Type and MinimalUnion64Type. These are defined the same as CompleteUnionType and MinimalUnion64Type, except that the discriminator labels are 64-bit integers instead of 32-bit integers.