[clangd] Add Source to clangd::Diagnostic.

Summary:
clangd embedder can distinguish diagnostics from clang or clang-tidy.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58600

llvm-svn: 355493