[clangd] IndexedFiles should include Fils from RefSlab and RelationSlab.

[hokein]

Summary

This looks like an oversight.

clang-tools-extra/clangd/index/FileIndex.cpp

FileSymbols::buildIndex(IndexType Type, DuplicateHandling DuplicateHandle,
   {
     std::lock_guard<std::mutex> Lock(Mutex);
     for (const auto &FileAndSymbols : SymbolsSnapshot) {
       SymbolSlabs.push_back(FileAndSymbols.second);
       Files.insert(FileAndSymbols.first());
     }
     for (const auto &FileAndRefs : RefsSnapshot) {
       RefSlabs.push_back(FileAndRefs.second.Slab);
+      Files.insert(FileAndRefs.first());
       if (FileAndRefs.second.CountReferences)
         MainFileRefs.push_back(RefSlabs.back().get());
     }
-    for (const auto &FileAndRelations : RelationsSnapshot)
+    for (const auto &FileAndRelations : RelationsSnapshot) {
+      Files.insert(FileAndRelations.first());
       RelationSlabs.push_back(FileAndRelations.second);
+    }
 
     if (Version)
       *Version = this->Version;
   }
   std::vector<const Symbol *> AllSymbols;
   std::vector<Symbol> SymsStorage;
   switch (DuplicateHandle) {
   case DuplicateHandling::Merge: {