swift234

IRGen: Preliminary support for resilient enums

Resilient enums are manipulated as opaque values.

Clients are still allowed to assume physical case indices and case
payload types for now -- we might add a level of indirection here,
which would require designing a new case dispatch mechanism.

Resilient enums are never constructed directly, only by calling
case constructor functions. Case constructors already get emitted,
however they're [transparent] -- this will change in a subsequent
patch.

We could save on code size by emitting an InjectEnumTag value
witness function that can construct any case given a physical case
number, rather than emitting constructors for each case, but for
now going through case constructor functions will suffice.