[clangd] Switch to Dex by default for the static index

Dex is now mature enough to be used as the default static index. This
patch performs the switch but introduces a hidden flag to allow users
fallback to Mem in case something happens.

Reviewed by: ioeric

Differential Revision: https://reviews.llvm.org/D51352

llvm-svn: 340828

clang-tools-extra/clangd/tool/ClangdMain.cpp

@@ -29,10 +29,11 @@
 using namespace clang;
 using namespace clang::clangd;
 
+// FIXME: remove this option when Dex is stable enough.
 static llvm::cl::opt<bool>
     UseDex("use-dex-index",
            llvm::cl::desc("Use experimental Dex static index."),
-           llvm::cl::init(false), llvm::cl::Hidden);
+           llvm::cl::init(true), llvm::cl::Hidden);
 
 namespace {