[clangd] Do not report anonymous entities in findExplicitReferences

Summary:
Otherwise every client dealing with name location should handle
anonymous names in a special manner.

This seems too error-prone, clients can probably handle anonymous
entities they care about differently.

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69511