[llvm-lipo] Use std::optional in llvm-lipo.cpp (NFC)

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

llvm/tools/llvm-lipo/llvm-lipo.cpp

@@ -31,6 +31,7 @@
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Support/WithColor.h"
 #include "llvm/TextAPI/Architecture.h"
+#include <optional>
 
 using namespace llvm;
 using namespace llvm::object;
@@ -105,7 +106,7 @@ enum class LipoAction {
 };
 
 struct InputFile {
-  Optional<StringRef> ArchType;
+  std::optional<StringRef> ArchType;
   StringRef FileName;
 };