[clangd] Simplify the code in Index::refs

Summary:
While here, also fix potential UB in MergeIndex.

Thanks Kadir for finding this!

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70225