[clangd] Pull installed gRPC and introduce clangd-remote-(server|client)

Summary:
This patch allows using installed gRPC to build two simple tools which
currently provide the functionality of looking up the symbol by name.
remote-index-client is a simplified version of dexp which connects to
remote-index-server passes lookup requests.

I also significantly reduced the scope of this patch to prevent large changelist
and more bugs. The next steps would be:

* Extending Protocol for deep copies of Symbol and inherit RemoteIndex from
  Index to unify the interfaces
* Make remote-index-server more generic and merge the remote index client with
  dexp
* Modify Clangd to allow using remote index instead of the local one for all
  global index requests

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: mgorny, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77794