[clangd] Set "spelled" flag for constructor references.

Summary:
DeclarationName for cxx constructor is special, it is not an identifier.
thus the "Spelled" flag are not set for all ctor references, this patch
fixes it.

Reviewers: kbobyrev

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74125