Revert "[mlir] Remove the unused source file."

This reverts commit e488ce2.

Reverted to fix a compilation issue on gcc8.

mlir/unittests/Dialect/Affine/Analysis/AffineStructuresParser.cpp

@@ -0,0 +1,25 @@
+//===- AffineStructuresParser.cpp - Parser for AffineStructures -*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "./AffineStructuresParser.h"
+#include "mlir/AsmParser/AsmParser.h"
+#include "mlir/IR/IntegerSet.h"
+
+using namespace mlir;
+using namespace presburger;
+
+FailureOr<IntegerPolyhedron>
+mlir::parseIntegerSetToFAC(llvm::StringRef str, MLIRContext *context,
+                           bool printDiagnosticInfo) {
+  IntegerSet set = parseIntegerSet(str, context, printDiagnosticInfo);
+
+  if (!set)
+    return failure();
+
+  return FlatAffineValueConstraints(set);
+}