[clangd] Refine the way of checking a declaration is referenced by the written code.

Summary:
The previous solution (checking the AST) is not a reliable way to
determine whether a declaration is explicitly referenced by the source
code, we are still missing a few cases.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D55191

llvm-svn: 349033