[Serialization] Fix one PrettyStackTrace, add another. (#7436)

We want to see /what/ types are failing here.

lib/Serialization/Serialization.cpp

@@ -20,8 +20,8 @@
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/Initializer.h"
 #include "swift/AST/LinkLibrary.h"
+#include "swift/AST/PrettyStackTrace.h"
 #include "swift/AST/ProtocolConformance.h"
-#include "swift/AST/ASTMangler.h"
 #include "swift/AST/RawComment.h"
 #include "swift/AST/USRGeneration.h"
 #include "swift/Basic/Dwarf.h"
@@ -465,7 +465,10 @@ TypeID Serializer::addTypeRef(Type ty) {
   if (!ty)
     return 0;
 
+#ifndef NDEBUG
+  PrettyStackTraceType trace(M->getASTContext(), "serializing", ty);
   assert(!ty->hasError() && "Serializing error type");
+#endif
 
   auto &id = DeclAndTypeIDs[ty];
   if (id.first != 0)