[clangd] Implement "textDocument/documentLink" protocol support

Summary:
This adds an implementation for the "textDocument/documentLink" LSP request.

It returns links for all `#include` directives to the resolved target files.

Fixes https://github.com/clangd/clangd/issues/217.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70872