[clangd] Extend dexp to support remote index

Summary:
* Merge clangd-remote-client into dexp
* Implement `clangd::remote::IndexClient` that is derived from `SymbolIndex`
* Upgrade remote mode-related CMake infrastructure

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78521