swift234

IRGen: Add ValueWitness enumerators for union layout and access.

These value witness table entries will be conditionally available for types that support specialized union representation through extra inhabitants and/or spare bits and for union value witnesses:

- storeExtraInhabitant, to store an extra inhabitant representation;
- getExtraInhabitantIndex, to recognize an extra inhabitant representation;
- getUnionTag, to get a union's discriminator; and
- inplaceProjectUnionData, to extract the value in place from a union.

This just sets up the enumerators and related IR types and mangling; nothing emits these witnesses yet.

Swift SVN r7234