[clangd] Publish xref for macros from Index and AST.

Summary:
With this patch the `findReferences` API will return Xref for macros.
If the symbol under the cursor is a macro then we collect the references to it from:
1. Main file by looking at the ParsedAST. (These were added to the ParsedAST in https://reviews.llvm.org/D70008)
2. Files other than the mainfile by looking at the:
	* static index (Added in https://reviews.llvm.org/D70489)
	* file index (Added in https://reviews.llvm.org/D71406)
This patch collects all the xref from the above places and outputs it in `findReferences` API.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72395