Add SemanticRanges to Clangd server.

Summary:
Adds Semantic Ranges capabilities to Clangd server.
Also adds tests for running it via clangd server.

This differs from the LSP spec as the spec needs this to be evaluated on multiple 'pos' and the expected output is an list of list of semantic ranges.
This is majorly for multi cursor and assuming this is a rare thing, we don't want to optimize make things complicated just for this.
This should be done in the LSP level by queueing one request per 'pos' in the input.

LSP 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/D67650

llvm-svn: 372102