[clang] Use std::nullopt instead of None in comments (NFC)

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

GitOrigin-RevId: 35b4fbb559d909a7edf64412c665e99748398ac4
35 files changed