[clangd] Add createIndex in dexp

Summary:
This would allow easily injecting our internal customization.

Also updates the stale "symbol-collection-file" flag.

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D53292

llvm-svn: 344521