Remove llvm::Optional

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/D149128

Author: kazutakahirata

llvm/include/llvm/ADT/None.h

@@ -18,9 +18,8 @@
 #ifndef MLIR_SUPPORT_LLVM_H
 #define MLIR_SUPPORT_LLVM_H
 
-// We include these two headers because they cannot be practically forward
+// We include this header because it cannot be practically forward
 // declared, and are effectively language features.
-#include "llvm/ADT/None.h"
 #include "llvm/Support/Casting.h"
 #include <vector>
 
@@ -58,7 +57,6 @@ class DenseSet;
 class MallocAllocator;
 template <typename T>
 class MutableArrayRef;
-template <typename T> using Optional = std::optional<T>;
 template <typename... PT>
 class PointerUnion;
 template <typename T, typename Vector, typename Set>
@@ -130,7 +128,6 @@ using SetVector = llvm::SetVector<T, Vector, Set>;
 template <typename AllocatorTy = llvm::MallocAllocator>
 using StringSet = llvm::StringSet<AllocatorTy>;
 using llvm::MutableArrayRef;
-using llvm::Optional;
 using llvm::PointerUnion;
 using llvm::SmallPtrSet;
 using llvm::SmallPtrSetImpl;