[clangd] Expose completion range in code completion results (C++ API)

Summary:
Informative only, useful for positioning UI, interacting with other sources of
completion etc. As requested by an embedder of clangd.

Reviewers: usaxena95

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74305