[clangd] Don't store completion info if the symbol is not used for code completion.

Summary:
This would save us some memory and disk space:
  - Dex usage (261 MB vs 266 MB)
  - Disk (75 MB vs 76 MB)

It would save more when we index the main file symbol D55185.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: nridge, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D56314

llvm-svn: 350803