[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