[clangd] Lay JSONRPCDispatcher to rest.

Summary:
Most of its functionality is moved into ClangdLSPServer.
The decoupling between JSONRPCDispatcher, ProtocolCallbacks, ClangdLSPServer
was never real, and only served to obfuscate.

Some previous implicit/magic stuff is now explicit:
 - the return type of LSP method calls are now in the signature
 - no more reply() that gets the ID using global context magic
 - arg tracing no longer relies on RequestArgs::stash context magic either

This is mostly refactoring, but some deliberate fixes while here:
 - LSP method params are now by const reference
 - notifications and calls are now distinct namespaces.
   (some tests had protocol errors and needed updating)
 - we now reply to calls we failed to decode
 - outgoing calls use distinct IDs
A few error codes and message IDs changed in unimportant ways (see tests).

Reviewers: ioeric

Subscribers: mgorny, ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, jfb, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53387

llvm-svn: 344737