[clang] Remove clang::Optional

[kazu]

Summary

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

[MaskRay]

Thanks!

clang/include/clang/Basic/LLVM.h

 
 #ifndef LLVM_CLANG_BASIC_LLVM_H
 #define LLVM_CLANG_BASIC_LLVM_H
 
 // Do not proliferate #includes here, require clients to #include their
 // dependencies.
 // Casting.h has complex templates that cannot be easily forward declared.
 #include "llvm/Support/Casting.h"
-// None.h includes an enumerator that is desired & cannot be forward declared
-// without a definition of NoneType.
-#include "llvm/ADT/None.h"
 // Add this header as a workaround to prevent `too few template arguments for
 // class template 'SmallVector'` building error with build compilers like XL.
 #include "llvm/ADT/SmallVector.h"
 
 namespace llvm {
   // ADT's.
   class StringRef;
   class Twine;
   class VersionTuple;
   template<typename T> class ArrayRef;
   template<typename T> class MutableArrayRef;
   template<typename T> class OwningArrayRef;
   template<unsigned InternalLen> class SmallString;
   template<typename T, unsigned N> class SmallVector;
   template<typename T> class SmallVectorImpl;
-  template <typename T> using Optional = std::optional<T>;
   template <class T> class Expected;
 
   template<typename T>
   struct SaveAndRestore;
 
   // Reference counting.
   template <typename T> class IntrusiveRefCntPtr;
   template <typename T> struct IntrusiveRefCntPtrInfo;
namespace clang {
   using llvm::dyn_cast_or_null;
   using llvm::dyn_cast_if_present;
   using llvm::cast_or_null;
   using llvm::cast_if_present;
 
   // ADT's.
   using llvm::ArrayRef;
   using llvm::MutableArrayRef;
-  using llvm::Optional;
   using llvm::OwningArrayRef;
   using llvm::SaveAndRestore;
   using llvm::SmallString;
   using llvm::SmallVector;
   using llvm::SmallVectorImpl;
   using llvm::StringRef;
   using llvm::Twine;
   using llvm::VersionTuple;