This will help with ensuring that we do not create multiple witness
table "definitions" one of which is null. That situtation yields an
IRGen assertion to be hit since the external declaration (in the guise
of a definition) has a different type from the actual deserialized
definition.
Swift SVN r14999
In the short term, we need to be able to emit shared symbols for SILWitnessTables corresponding to Clang-imported modules, and soon, the generic specializer will need to be able to reference *_external witness tables deserialized from library modules.
Swift SVN r14887
This will enable the creation of external witness tables whose entries
can be initialized after the witness table itself has been constructed.
This can occur if the table is deserialized later from a different
module.
Swift SVN r14715
Tweak the type lowering code to work when the conforming type is generic. Handle the case of an associated type with protocol requirements being witnessed by an archetype of the conforming type, which results in a null ProtocolConformance pointer in the witnessing substitution.
Swift SVN r11275
When a type conforms to a protocol that refines another protocol, emit the witness table for the base protocol, and drop a reference into the witness table for the derived protocol. Keep track of what conformances we've already emitted so we don't emit redundant witness tables when types conform redundantly to base protocols or have multiple references to a base protocol via a refinement diamond.
Swift SVN r11263
Walk the ProtocolConformances of type and extension decls to produce SILWitnessTables for them. Work out the type of the witness function by applying substitutions from the witness map and lowering it at the abstraction level of the requirement, then emit a symbol for the witness function (but don't emit the body of the witness function just yet).
Swift SVN r11143
We will generate these in SILGen when we see a NormalProtocolConformance, to provide a mapping of method requirements to witnesses for types.
Swift SVN r10900
