[clangd] Add semantic selection to ClangdLSPServer.

[clangd] Add semantic selection to ClangdLSPServer.

Summary:
This adds semantic selection to the LSP Server.
Adds support for serialization of input request and the output reply.
Also adds regression tests for the feature.

Currently we do not support multi cursor.The LSP Server only accepts single position in the request as opposed to many position in the spec.

Spec:
https://github.com/microsoft/language-server-protocol/blob/dbaeumer/3.15/specification.md#textDocument_selectionRange

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67720

llvm-svn: 372753