[ADT] Define None as std::nullopt (NFC)

This patch defines NoneType and None as std::nullopt_t and
std::nullopt, respectively.

This patch effectively makes None and std::nullopt interchangeable, so
we can gradually replace None with std::nullopt all while we continue
to use llvm::Optional.

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/D138468