[clangd] Make USRs for macros to be position independent

Summary:
USRs for macros were not cannonical due to usage of cursor location
instead of definition location.

Reviewers: jkorous

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

Differential Revision: https://reviews.llvm.org/D57228

llvm-svn: 352481