This patch replaces NoneType() and NoneType::None with None in
preparation for migration from llvm::Optional to std::optional.
In the std::optional world, we are not guranteed to be able to
default-construct std::nullopt_t or peek what's inside it, so neither
NoneType() nor NoneType::None has a corresponding expression in the
std::optional world.
Once we consistently use None, we should even be able to replace the
contents of llvm/include/llvm/ADT/None.h with something like:
using NoneType = std::nullopt_t;
inline constexpr std::nullopt_t None = std::nullopt;
to ease the migration from llvm::Optional to std::optional.
Differential Revision: https://reviews.llvm.org/D138376
1fa870b1bd