[clangd] Add bool return type to Index::refs API.

Summary:
Similar to fuzzyFind, the bool indicates whether there are more xref
results.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70139