swift234

Reflection: Multi-payload enum layout

The approach here is to split this into two cases:

- If all case payloads have a fixed size, spare bits may be
  potentially used to differentiate between cases, and the
  remote reflection library does not have enough information to
  compute the layout itself.

  However, the total size must be fixed, so IRGen just emits a
  builtin type descriptor (which I need to rename to 'fixed type
  descriptor' since these are also used for imported value types,
  and now, certain enums).

- If at least one case has a size that depends on a generic
  parameter or is a resilient type, IRGen does not know the size,
  but this means fancy tricks with spare bits cannot be used either.
  The remote reflection library uses the same approach as the
  runtime, basically taking the maximum of the payload size and
  alignment, and adding a tag byte.

As with single-payload enums, we produce a new kind of
RecordTypeInfo, this time with a field for every enum case.
All cases start at offset zero (but of course this might change,
if for example we put the enum tag before the address point).

Also, just as with single-payload enums, there is no remote
'project case index' operation on ReflectionContext yet.

So the the main benefit from this change is that we don't entirely
give up when doing layout of class instances containing enums;
however, tools still cannot look inside the enum values themselves,
except in the simplest cases involving optionals.

Notably, the remote reflection library finally understands all
of the standard library's collection types -- Array, Character,
Dictionary, Set, and String.