[clangd-remote] Replace YAML serialization with proper Protobuf messages

Summary:
YAML serialization was used in the Proof of Concept for simplicity.
This patch replaces implements Protobuf (de) serialization of almost all
types that need to be transferred over the protocol.

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79862