[clangd] Use name of Macro to compute its SymbolID, NFC.

Summary:
We use the name from the IdentifierInfo of the Macro to compute its
SymbolID. It is better to just take the Name as a parameter to avoid
storing the IdentifierInfo whenever we need the SymbolID for the Macro.

Patch by UTKARSH SAXENA!

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69937