Apply clang-tidy fixes for misc-include-cleaner in AST.cpp (NFC)

Author: joker-eph

mlir/examples/toy/Ch1/parser/AST.cpp

@@ -12,9 +12,12 @@
 
 #include "toy/AST.h"
 
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/ADT/TypeSwitch.h"
+#include "llvm/Support/Casting.h"
 #include "llvm/Support/raw_ostream.h"
+#include <string>
 
 using namespace toy;