[clangd] Use expansion location when the ref is inside macros.
Previously, xrefs has inconsistent behavior when the reference is inside
- 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%.
Subscribers: merge_guards_bot, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits
Differential Revision: https://reviews.llvm.org/D70480