[FileCheck] Use std::optional in FileCheck.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/lib/FileCheck/FileCheck.cpp

@@ -22,6 +22,7 @@
 #include "llvm/Support/FormatVariadic.h"
 #include <cstdint>
 #include <list>
+#include <optional>
 #include <set>
 #include <tuple>
 #include <utility>
@@ -695,7 +696,7 @@ Pattern::parseCallExpr(StringRef &Expr, StringRef FuncName,
   Expr = Expr.ltrim(SpaceChars);
   assert(Expr.startswith("("));
 
-  auto OptFunc = StringSwitch<Optional<binop_eval_t>>(FuncName)
+  auto OptFunc = StringSwitch<std::optional<binop_eval_t>>(FuncName)
                      .Case("add", operator+)
                      .Case("div", operator/)
                      .Case("max", max)