[clangd] less boilerplate in RPC dispatch

Summary:
Make the ProtocolHandlers glue between JSONRPCDispatcher and
ClangdLSPServer generic.
Eliminate small differences between methods, de-emphasize the unimportant
distinction between notifications and methods.

ClangdLSPServer is no longer responsible for producing a complete
JSON-RPC response, just the JSON of the result object. (In future, we
should move that JSON serialization out, too).
Handler methods now take a context object that we may hang more
functionality off in the future.

Added documentation to ProtocolHandlers.

Reviewers: ilya-biryukov, bkramer

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38464

llvm-svn: 315577