SILGen: Lazily emit ClangImporter-synthesized conformances even when they're incomplete

Currently Sema completes all _ObjectiveCBridgeable conformances it thinks
are needed in SILGen and runtime dynamic casts, but that's about to change.

When emitting conformances for an imported type we would skip incomplete
conformances. Now that we have a long-lived type checker instance, there
is no longer any reason to do that. Looking up witnesses from an
incomplete conformance will trigger conformance checking as needed.

Furthermore, conformances emitted in this path are emitted lazily, so if
a conformance is not used, no conformance checking will take place.

A change in behavior only occurs after subsequent changes to remove Sema's
eager checking of ClangImporter-synthesized _ObjectiveCBridgeable
conformances; at that point, we must be prepared to lazily emit these
incomplete conformances.

Author: slavapestov

lib/SILGen/SILGenDecl.cpp

@@ -1440,9 +1440,9 @@ void SILGenModule::emitExternalDefinition(Decl *d) {
                                             nullptr)) {
       auto *proto = c->getProtocol();
       if (Lowering::TypeConverter::protocolRequiresWitnessTable(proto) &&
-          isa<NormalProtocolConformance>(c) &&
-          c->isComplete())
+          isa<NormalProtocolConformance>(c)) {
         emitExternalWitnessTable(cast<NormalProtocolConformance>(c));
+      }
     }
     break;
   }