[NFC] Add ArrayRef includes to two files.

[stellaraccident]

Summary

These started failing on one of our buildbots. I didn't completely root cause the situation and just added the explicit includes that correct the issue.

mlir/include/mlir/IR/AffineExprVisitor.h

 //===- AffineExprVisitor.h - MLIR AffineExpr Visitor Class ------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file defines the AffineExpr visitor class.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef MLIR_IR_AFFINE_EXPR_VISITOR_H
 #define MLIR_IR_AFFINE_EXPR_VISITOR_H
 
 #include "mlir/IR/AffineExpr.h"
+#include "llvm/ADT/ArrayRef.h"
 
 namespace mlir {
 
 /// Base class for AffineExpr visitors/walkers.
 ///
 /// AffineExpr visitors are used when you want to perform different actions
 /// for different kinds of AffineExprs without having to use lots of casts
 /// and a big switch instruction.

mlir/include/mlir/Interfaces/FoldInterfaces.h

 //===- FoldInterfaces.h - Folding Interfaces --------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 #ifndef MLIR_INTERFACES_FOLDINTERFACES_H_
 #define MLIR_INTERFACES_FOLDINTERFACES_H_
 
 #include "mlir/IR/DialectInterface.h"
 #include "mlir/Support/LogicalResult.h"
+#include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallVector.h"
 
 namespace mlir {
 class Attribute;
 class OpFoldResult;
 class Region;
 
 /// Define a fold interface to allow for dialects to control specific aspects