[clangd] Add "Deprecated" field to Symbol and CodeCompletion.

Summary: Also set "deprecated" field in LSP CompletionItem.

Reviewers: sammccall, kadircet

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D51724

llvm-svn: 341576