[clangd] Use Dex for dynamic index by default.

Summary:
Memory usage for a sample TU:
  Without Dex: 17.9M
  With    Dex: 24.4M

The memory increase is considerable but seems tolerable.

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57878

llvm-svn: 353413
diff --git a/clang-tools-extra/clangd/tool/ClangdMain.cpp b/clang-tools-extra/clangd/tool/ClangdMain.cpp
index 6c102ed..9495869 100644
--- a/clang-tools-extra/clangd/tool/ClangdMain.cpp
+++ b/clang-tools-extra/clangd/tool/ClangdMain.cpp
@@ -31,7 +31,7 @@
 static llvm::cl::opt<bool>
     UseDex("use-dex-index",
            llvm::cl::desc("Use experimental Dex dynamic index."),
-           llvm::cl::init(false), llvm::cl::Hidden);
+           llvm::cl::init(true), llvm::cl::Hidden);
 
 static llvm::cl::opt<Path> CompileCommandsDir(
     "compile-commands-dir",