[clangd] Fix async index loading (from r341376).

[sammccall]

Summary

This wasn't actually async (due to std::future destructor blocking).
If it were, we would have clean shutdown issues if main returned
and destroyed Placeholder before the thread is done with it.

We could attempt to avoid any blocking by using shared_ptr or weak_ptr tricks so
the thread can detect Placeholder's destruction, but there are other potential
issues (e.g. loadIndex does tracing, and we'll destroy the tracer...)
Instead, once LSPServer::run returns, we wait for the index to finish loading
before exiting. Performance is not critical in this situation.

[sammccall]

Don't load index asynchronously if -run-synchronously is passed.

Nothing needs this today, but it's less surprising behavior.

[ilya-biryukov]

LGTM

clangd/tool/ClangdMain.cpp

int main(int argc, char *argv[]) {
   case PCHStorageFlag::Disk:
     Opts.StorePreamblesInMemory = false;
     break;
   }
   if (!ResourceDir.empty())
     Opts.ResourceDir = ResourceDir;
   Opts.BuildDynamicSymbolIndex = EnableIndex;
   std::unique_ptr<SymbolIndex> StaticIdx;
+  std::future<void> AsyncIndexLoad; // Block exit while loading the index.
   if (EnableIndex && !YamlSymbolFile.empty()) {
     // Load the index asynchronously. Meanwhile SwapIndex returns no results.
     SwapIndex *Placeholder;
     StaticIdx.reset(Placeholder = new SwapIndex(llvm::make_unique<MemIndex>()));
-    runAsync<void>([Placeholder] {
+    AsyncIndexLoad = runAsync<void>([Placeholder] {
       if (auto Idx = loadIndex(YamlSymbolFile))
         Placeholder->reset(std::move(Idx));
     });
   }
   Opts.StaticIndex = StaticIdx.get();
   Opts.AsyncThreadsCount = WorkerThreadsCount;
 
   clangd::CodeCompleteOptions CCOpts;