[deserialization] Enable lazy deserialization of witness tables.

This patch adds in the necessary infrastructure for lazily deserializing
witness tables. This is done by following the same approach as the
deserialization/serialization of SILFunction.

Now if one calls SILModule::lookUpWitnessTable and the given witness table is a
definition, the SILModule will attempt to deserialize it from one of the other
modules.

Swift SVN r15403

lib/Serialization/SerializedSILLoader.cpp

@@ -58,6 +58,13 @@ SILVTable *SerializedSILLoader::lookupVTable(Identifier Name) {
   return nullptr;
 }
 
+SILWitnessTable *SerializedSILLoader::lookupWitnessTable(SILWitnessTable *WT) {
+  for (auto &Des : LoadedSILSections)
+    if (auto wT = Des->lookupWitnessTable(WT))
+      return wT;
+  return nullptr;
+}
+
 void SerializedSILLoader::getAll() {
   for (auto &Des : LoadedSILSections)
     Des->getAll();