[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/SIL/IR/SILModule.cpp

@@ -115,6 +115,9 @@ SILModule::~SILModule() {
   for (SILGlobalVariable &v : silGlobals)
     v.dropAllReferences();
 
+  for (auto vt : vtables)
+    vt->~SILVTable();
+
   // Drop everything functions in this module reference.
   //
   // This is necessary since the functions may reference each other.  We don't