IRGen: Don't deserialize witness tables accidentally

This is actually NFC: We should have already deserialized everything
we need at this point, and because of large loadable types and
address lowering, deserializing more stuff in IRGen is not valid,
and in fact we check for this and refuse to deserialize.

Author: slavapestov

lib/IRGen/IRGenModule.cpp

@@ -769,7 +769,7 @@ bool IRGenerator::canEmitWitnessTableLazily(SILWitnessTable *wt) {
 }
 
 void IRGenerator::addLazyWitnessTable(const ProtocolConformance *Conf) {
-  if (SILWitnessTable *wt = SIL.lookUpWitnessTable(Conf)) {
+  if (auto *wt = SIL.lookUpWitnessTable(Conf, /*deserializeLazily=*/false)) {
     // Add it to the queue if it hasn't already been put there.
     if (canEmitWitnessTableLazily(wt) &&
         LazilyEmittedWitnessTables.insert(wt).second) {