[SIL] NFC: Simplify SILVTable and save 8 bytes per SILVTable

We were not using the primary benefits of an intrusive list, namely the
ability to insert or remove from the middle of the list, so let's switch
to a plain vector. This also avoids linked-list pointer chasing.

lib/Serialization/SerializeSIL.cpp

@@ -2659,10 +2659,10 @@ void SILSerializer::writeSILBlock(const SILModule *SILMod) {
   // Go through all SILVTables in SILMod and write them if we should
   // serialize everything.
   // FIXME: Resilience: could write out vtable for fragile classes.
-  for (const SILVTable &vt : SILMod->getVTables()) {
-    if ((ShouldSerializeAll || vt.isSerialized()) &&
-        SILMod->shouldSerializeEntitiesAssociatedWithDeclContext(vt.getClass()))
-      writeSILVTable(vt);
+  for (const auto &vt : SILMod->getVTables()) {
+    if ((ShouldSerializeAll || vt->isSerialized()) &&
+        SILMod->shouldSerializeEntitiesAssociatedWithDeclContext(vt->getClass()))
+      writeSILVTable(*vt);
   }
   
   // Write out property descriptors.