[mlir] Add #include <optional> (NFC)

This patch adds #include <optional> to those files containing
llvm::Optional<...> or Optional<...>.

I'll post a separate patch to actually replace llvm::Optional with
std::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

Author: kazutakahirata

mlir/examples/toy/Ch1/include/toy/AST.h

@@ -22,6 +22,7 @@
 #include "llvm/Support/Casting.h"
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace toy {
 

mlir/examples/toy/Ch1/include/toy/Parser.h

@@ -25,6 +25,7 @@
 #include <map>
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace toy {
 

mlir/examples/toy/Ch2/include/toy/AST.h

@@ -22,6 +22,7 @@
 #include "llvm/Support/Casting.h"
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace toy {
 

mlir/examples/toy/Ch2/include/toy/Parser.h

@@ -25,6 +25,7 @@
 #include <map>
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace toy {
 

mlir/examples/toy/Ch3/include/toy/AST.h

@@ -22,6 +22,7 @@
 #include "llvm/Support/Casting.h"
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace toy {
 

mlir/examples/toy/Ch3/include/toy/Parser.h

@@ -25,6 +25,7 @@
 #include <map>
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace toy {
 

mlir/examples/toy/Ch4/include/toy/AST.h

@@ -22,6 +22,7 @@
 #include "llvm/Support/Casting.h"
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace toy {
 

mlir/examples/toy/Ch4/include/toy/Parser.h

@@ -25,6 +25,7 @@
 #include <map>
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace toy {
 

mlir/examples/toy/Ch5/include/toy/AST.h

@@ -22,6 +22,7 @@
 #include "llvm/Support/Casting.h"
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace toy {
 

mlir/examples/toy/Ch5/include/toy/Parser.h

@@ -25,6 +25,7 @@
 #include <map>
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace toy {