[clangd] Add marshalling code for all request types

Summary:
Only FuzzyFindRequest is implemented via Marshaller even though other requests
also follow a similar pattern. Unify them under the marshalling umbrella and
make the server requests even more uniform to complement D84499.

Reviewers: kadircet

Reviewed By: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84525