[clangd] Use expansion location when the ref is inside macros.

Summary:
Previously, xrefs has inconsistent behavior when the reference is inside
macro body:
- AST-based xrefs (for main file) uses the expansion location;
- our index uses the spelling location;

This patch makes our index use file locations for references, which is
consistent with AST-based xrefs, and kythe as well.

After this patch, memory usage of static index on LLVM increases ~5%.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70480