[clangd] A helper to find explicit references and their names

Summary:
Allows to simplify pending code tweaks:
  - the upcoming DefineInline tweak (D66647)
  - remove using declaration (D56612)
  - qualify name under cursor (D56610)

Another potential future application is simplifying semantic highlighting.

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: mgrang, jfb, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67826

llvm-svn: 372859