[flang] Fixed Windows build.

`llvm::enumerate(llvm::reverse(ValueRange))` added in llvm#124683 does not work
on Windows: https://lab.llvm.org/buildbot/#/builders/124/builds/322

Author: vzakhari

flang/lib/Optimizer/HLFIR/Transforms/SimplifyHLFIRIntrinsics.cpp

@@ -1094,17 +1094,18 @@ class ReshapeAsElementalConversion
     mlir::Value zero = builder.createIntegerConstant(loc, indexType, 0);
     mlir::Value one = builder.createIntegerConstant(loc, indexType, 1);
     mlir::Value linearIndex = zero;
-    for (auto idx : llvm::enumerate(llvm::reverse(indices))) {
+    std::size_t idx = 0;
+    for (auto index : llvm::reverse(indices)) {
       mlir::Value tmp = builder.create<mlir::arith::SubIOp>(
-          loc, builder.createConvert(loc, indexType, idx.value()), one);
+          loc, builder.createConvert(loc, indexType, index), one);
       tmp = builder.create<mlir::arith::AddIOp>(loc, linearIndex, tmp);
-      if (idx.index() + 1 < rank)
+      if (idx + 1 < rank)
         tmp = builder.create<mlir::arith::MulIOp>(
             loc, tmp,
-            builder.createConvert(loc, indexType,
-                                  extents[rank - idx.index() - 2]));
+            builder.createConvert(loc, indexType, extents[rank - idx - 2]));
 
       linearIndex = tmp;
+      ++idx;
     }
     return linearIndex;
   }