[ThinLTO] Augment FunctionImport dump with value name to GUID map

[tejohnson]

Summary

To aid in debugging, dump out the correlation between value names and
GUID for each source module when it is materialized. This will make it
easier to comprehend the earlier summary-based function importing debug
trace which only has access to and prints the GUIDs.

[mehdi_amini]

LGTM

lib/Transforms/IPO/FunctionImport.cpp

for (auto &Name : ModuleNameOrderedList) {
     // now, before linking it (otherwise this will be a noop).
     SrcModule->materializeMetadata();
     UpgradeDebugInfo(*SrcModule);
 
     auto &ImportGUIDs = FunctionsToImportPerModule->second;
     // Find the globals to import
     DenseSet<const GlobalValue *> GlobalsToImport;
     for (auto &GV : *SrcModule) {
-      if (GV.hasName() && ImportGUIDs.count(GV.getGUID())) {
+      if (!GV.hasName())
+        continue;
+      auto GUID = GV.getGUID();
+      auto Import = ImportGUIDs.count(GUID);
+      DEBUG(dbgs() << (Import ? "Is" : "Not") << " importing " << GUID << " "
+                   << GV.getName() << " from " << SrcModule->getSourceFileName()
+                   << "\n");
+      if (Import) {
         GV.materialize();
         GlobalsToImport.insert(&GV);
       }
     }
     for (auto &GV : SrcModule->aliases()) {
       if (!GV.hasName())
         continue;
       auto GUID = GV.getGUID();
-      if (ImportGUIDs.count(GUID)) {
+      auto Import = ImportGUIDs.count(GUID);
+      DEBUG(dbgs() << (Import ? "Is" : "Not") << " importing " << GUID << " "
+                   << GV.getName() << " from " << SrcModule->getSourceFileName()
+                   << "\n");
+      if (Import) {
         // Alias can't point to "available_externally". However when we import
         // linkOnceODR the linkage does not change. So we import the alias
         // and aliasee only in this case.
         const GlobalObject *GO = GV.getBaseObject();
         if (!GO->hasLinkOnceODRLinkage())
           continue;
         GV.materialize();
         GlobalsToImport.insert(&GV);
         GlobalsToImport.insert(GO);
       }
     }
     for (auto &GV : SrcModule->globals()) {
       if (!GV.hasName())
         continue;
       auto GUID = GV.getGUID();
-      if (ImportGUIDs.count(GUID)) {
+      auto Import = ImportGUIDs.count(GUID);
+      DEBUG(dbgs() << (Import ? "Is" : "Not") << " importing " << GUID << " "
+                   << GV.getName() << " from " << SrcModule->getSourceFileName()
+                   << "\n");
+      if (Import) {
         GV.materialize();
         GlobalsToImport.insert(&GV);
       }
     }
 
     // Link in the specified functions.
     if (renameModuleForThinLTO(*SrcModule, Index, &GlobalsToImport))
       return true;