[clangd] Bump index version and get rid of wrong assertion

[kadircet]

Summary

After rL360344, BackgroundIndex expects symbols with zero refcounts.
Therefore existing index files are no longer valid.

Assertion regarding finding target of a reference was wrong, since
background-index might still be going on or we might've loaded only some part of
the shards and might be missing the declaring shards for the symbol.

[ilya-biryukov]

LGTM

clangd/index/FileIndex.cpp

for (const auto &Slab : SymbolSlabs) {
         auto I = Merged.try_emplace(Sym.ID, Sym);
         if (!I.second)
           I.first->second = mergeSymbol(I.first->second, Sym);
       }
     }
     for (const RefSlab *Refs : MainFileRefs)
       for (const auto &Sym : *Refs) {
         auto It = Merged.find(Sym.first);
-        assert(It != Merged.end() && "Reference to unknown symbol");
+        // This might happen while background-index is still running.
+        if (It == Merged.end())
+          continue;
         It->getSecond().References += Sym.second.size();
       }
     SymsStorage.reserve(Merged.size());
     for (auto &Sym : Merged) {
       SymsStorage.push_back(std::move(Sym.second));
       AllSymbols.push_back(&SymsStorage.back());
     }
     break;

clangd/index/Serialization.cpp

 //   - srcs: information related to include graph
 //   - stri: string table
 //   - symb: symbols
 //   - refs: references to symbols
 
 // The current versioning scheme is simple - non-current versions are rejected.
 // If you make a breaking change, bump this version number to invalidate stored
 // data. Later we may want to support some backward compatibility.
-constexpr static uint32_t Version = 9;
+constexpr static uint32_t Version = 10;
 
 llvm::Expected<IndexFileIn> readRIFF(llvm::StringRef Data) {
   auto RIFF = riff::readFile(Data);
   if (!RIFF)
     return RIFF.takeError();
   if (RIFF->Type != riff::fourCC("CdIx"))
     return makeError("wrong RIFF type");
   llvm::StringMap<llvm::StringRef> Chunks;