[clangd] Do not store BaseOf relations for missing subjects

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

for (const auto &SymRefs : *Index.Refs) {
         RefToSymID[&R] = SymRefs.first;
       }
     }
   }
   // Attribute relations to the file declaraing their Subject as Object might
   // not have been indexed, see SymbolCollector::processRelations for details.
   if (Index.Relations) {
     for (const auto &R : *Index.Relations) {
-      auto *File = SymbolIDToFile.lookup(R.Subject);
-      assert(File && "unknown subject in relation");
+      // FIXME: RelationSlab shouldn't contain dangling relations.
+      if (auto *File = SymbolIDToFile.lookup(R.Subject))
         File->Relations.insert(&R);
     }
   }
   // Store only the direct includes of a file in a shard.
   if (Index.Sources) {
     const auto &FullGraph = *Index.Sources;
     for (const auto &It : FullGraph) {
       auto File = UriToFile[It.first()];
       auto ShardIt = Shards.try_emplace(File);

clang-tools-extra/clangd/unittests/FileIndexTests.cpp

IndexFileIn IF;
     IF.Refs = std::move(*refSlab(Sym1.ID, BSourceUri.c_str()).release());
   }
   {
     RelationSlab::Builder B;
     // Should be stored in a.h
     B.insert(Relation{Sym1.ID, RelationKind::BaseOf, Sym2.ID});
     // Should be stored in b.h
     B.insert(Relation{Sym2.ID, RelationKind::BaseOf, Sym1.ID});
+    // Dangling relation should be dropped.
+    B.insert(Relation{symbol("3").ID, RelationKind::BaseOf, Sym1.ID});
     IF.Relations = std::move(B).build();
   }
 
   IF.Sources.emplace();
   IncludeGraph &IG = *IF.Sources;
   {
     // b.cc includes b.h
     auto &Node = IG[BSourceUri];