The addition of the helper is split out from https://reviews.llvm.org/D69543
as suggested by Kadir. I also updated the existing uses to use the new API.
This patch adds the cross references for Macros in the MainFile.
We add references for the main file to the ParsedAST. We query the
references from it using the SymbolID.
Xref outside main file will be added to the index in a separate patch.