[ADT] Deperecate llvm::None

[kazu]

Summary

I've converted all known uses of None to std::nullopt.

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

[dblaikie]

Great, thanks!

llvm/include/llvm/ADT/None.h

 #include "llvm/Support/Compiler.h"
 #include <optional>
 
 namespace llvm {
 /// A simple null object to allow implicit construction of Optional<T>
 /// and similar types without having to spell out the specialization's name.
 LLVM_DEPRECATED("Use std::nullopt_t instead", "std::nullopt_t")
 typedef std::nullopt_t NoneType;
+LLVM_DEPRECATED("Use std::nullopt instead.", "std::nullopt")
 inline constexpr std::nullopt_t None = std::nullopt;
 }
 
 #endif