[clangd] Fix early selection for non-vardecl declarators

Summary:
Selection tree was performing an early claim only for VarDecls, but
there are other cases where we can have declarators, e.g. FieldDecls. This patch
extends the early claim logic to all types of declarators.

Fixes https://github.com/clangd/clangd/issues/292

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75106