[clangd] Introduce a structured hover response

Summary:
Change ClangdServer layer to output a structured response for Hover,
which can be rendered by client according to their needs.

Reviewers: sammccall, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61497

llvm-svn: 361803