[clangd] Support relatedInformation in diagnostics.

Summary: We already have the structure internally, we just need to expose it.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60267

llvm-svn: 358675