[clangd] Use Dex for dynamic index by default.

[ioeric]

Summary

Memory usage for a sample TU:

Without Dex: 17.9M
With    Dex: 24.4M

The memory increase is considerable but seems tolerable.

[sammccall]

(Just flipping the flag seems fine too)

[ioeric]

• Keep the option for now.

clangd/tool/ClangdMain.cpp

 #include <cstdlib>
 #include <iostream>
 #include <memory>
 #include <string>
 #include <thread>
 
 namespace clang {
 namespace clangd {
-// FIXME: remove this option when Dex is cheap enough.
-static llvm::cl::opt<bool>
-    UseDex("use-dex-index",
-           llvm::cl::desc("Use experimental Dex dynamic index."),
-           llvm::cl::init(false), llvm::cl::Hidden);
 
 static llvm::cl::opt<Path> CompileCommandsDir(
     "compile-commands-dir",
     llvm::cl::desc("Specify a path to look for compile_commands.json. If path "
                    "is invalid, clangd will look in the current directory and "
                    "parent paths of each source file."));
 
 static llvm::cl::opt<unsigned>
case PCHStorageFlag::Memory:
     break;
   case PCHStorageFlag::Disk:
     Opts.StorePreamblesInMemory = false;
     break;
   }
   if (!ResourceDir.empty())
     Opts.ResourceDir = ResourceDir;
   Opts.BuildDynamicSymbolIndex = EnableIndex;
-  Opts.HeavyweightDynamicSymbolIndex = UseDex;
+  Opts.HeavyweightDynamicSymbolIndex = true;
   Opts.BackgroundIndex = EnableBackgroundIndex;
   Opts.BackgroundIndexRebuildPeriodMs = BackgroundIndexRebuildPeriod;
   std::unique_ptr<SymbolIndex> StaticIdx;
   std::future<void> AsyncIndexLoad; // Block exit while loading the index.
   if (EnableIndex && !IndexFile.empty()) {
     // Load the index asynchronously. Meanwhile SwapIndex returns no results.
     SwapIndex *Placeholder;
     StaticIdx.reset(Placeholder = new SwapIndex(llvm::make_unique<MemIndex>()));