[clangd] Expose qualified symbol names in CompletionItem (C++ structure only, no json).

Summary:
The qualified name can be used to match a completion item to its corresponding
symbol. This can be useful for tools that measure code completion quality.
Qualified names are not precise for identifying symbols; we need to figure out a
better way to identify completion items.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D48425

llvm-svn: 335334