[clangd] Add xref for macro to static index.

Summary:
This adds the references for macros to the SymbolCollector (used for static index).
Enabled if `CollectMacro` option is set.

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70489