[clangd] Collect name references in the index.

Summary:
This is used for cross-file rename. When renaming a class, we expect to
rename all related constructors/destructors.

Reviewers: kadircet, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69338