[Lex/HeaderSearch] Only lookup `HeaderFileInfo` for a `FileEntry` from `ExternalSource` once

[akyrtzi]

Summary

Without marking the HeaderFileInfo as resolved the FileEntries of files that were not found in ExternalSource
are looked up in ExternalSource over and over again.

clang/lib/Lex/HeaderSearch.cpp

 /// FileEntry.
 HeaderFileInfo &HeaderSearch::getFileInfo(const FileEntry *FE) {
   if (FE->getUID() >= FileInfo.size())
     FileInfo.resize(FE->getUID() + 1);
 
   HeaderFileInfo *HFI = &FileInfo[FE->getUID()];
   // FIXME: Use a generation count to check whether this is really up to date.
   if (ExternalSource && !HFI->Resolved) {
+    HFI->Resolved = true;
     auto ExternalHFI = ExternalSource->GetHeaderFileInfo(FE);
     if (ExternalHFI.IsValid) {
-      HFI->Resolved = true;
       if (ExternalHFI.External)
         mergeHeaderFileInfo(*HFI, ExternalHFI);
     }
   }
 
   HFI->IsValid = true;
   // We have local information about this header file, so it's no longer
   // strictly external.
if (FE->getUID() >= FileInfo.size()) {
         return nullptr;
       FileInfo.resize(FE->getUID() + 1);
     }
 
     HFI = &FileInfo[FE->getUID()];
     if (!WantExternal && (!HFI->IsValid || HFI->External))
       return nullptr;
     if (!HFI->Resolved) {
+      HFI->Resolved = true;
       auto ExternalHFI = ExternalSource->GetHeaderFileInfo(FE);
       if (ExternalHFI.IsValid) {
-        HFI->Resolved = true;
         if (ExternalHFI.External)
           mergeHeaderFileInfo(*HFI, ExternalHFI);
       }
     }
   } else if (FE->getUID() >= FileInfo.size()) {
     return nullptr;
   } else {
     HFI = &FileInfo[FE->getUID()];