Return None instead of Optional<T>() (NFC)

This patch replaces:

  return Optional<T>();

with:

  return None;

to make the migration from llvm::Optional to std::optional easier.
Specifically, I can deprecate None (in my source tree, that is) to
identify all the instances of None that should be replaced with
std::nullopt.

Note that "return None" far outnumbers "return Optional<T>();".  There
are more than 2000 instances of "return None" in our source tree.

All of the instances in this patch come from functions that return
Optional<T> except Archive::findSym and ASTNodeImporter::import, where
we return Expected<Optional<T>>.  Note that we can construct
Expected<Optional<T>> from any parameter convertible to Optional<T>,
which None certainly is.

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/D138464