swift234

IRGen: Emit remote reflection metadata for @objc protocols

I noticed even with multi-payload enum layout working, remote
reflection could not understand Set and Dictionary, because of
issues with Objective-C protocols in the standard library that
are used in the representation of those types.

The problem is that we were taking a different code path and
not emitting a field descriptor. A field descriptor is necessary
to differentiate Objective-C, class-bound and opaque protocols
from each other, so that existentials can use the correct
representation.