[clangd] Add a callback mechanism for handling responses from client.

Summary:
The callback will be invoked in clangd when we receive a reply from the client.

This is a prerequisite of implementing a generic mechanism for chainable
refactorings (e.g. extract variable and rename), this would allow server to
trigger a new request to the LSP client after receiving a reply from the client.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65387

llvm-svn: 367845