Use std::nullopt_t instead of NoneType (NFC)

This patch replaces those occurrences of NoneType that would trigger
an error if the definition of NoneType were missing in None.h.

To keep this patch focused, I am deliberately not replacing None with
std::nullopt in this patch or updating comments.  They will be
addressed in subsequent patches.

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

Differential Revision: https://reviews.llvm.org/D138539