[ADT] Deprecate llvm::NoneType

I've migrated all known uses of NoneType to std::nullopt_t.  This
patch deprecates NoneType.

I'm using "typedef" instead of "using" because somehow
"[[deprecated]]" and "using" do not seem to get along.

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/D138623