[clangd] Refactor ProtocolHandlers to decouple them from ClangdLSPServer

Summary:
A refactoring to decouple ProtocolHandlers and Language Server input parsing
loop from the ClangdLSPServer.
The input parsing was extracted from `main` to a function(runLanguageServerLoop).
ProtocolHandlers now provide an interface to handle various LSP methods,
this interface is used by ClangdLSPServer.
Methods for code formatting were moved from ProtocolHandlers to ClangdServer.
ClangdLSPServer now provides a cleaner interface that only runs Language Server
input loop.

Reviewers: bkramer, krasimir

Reviewed By: krasimir

Subscribers: cfe-commits, klimek

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D33201

llvm-svn: 303173