[Driver] Rename AddGoldPlugin to addLTOOptions. NFC

AddGoldPlugin does more than adding `-plugin path/to/LLVMgold.so`.
It works with lld and GNU ld, and adds other LTO options.
So AddGoldPlugin is no longer a suitable name.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D74591