[ADT][mlir][NFCI] Do not use non-const lvalue-refs with enumerate

Replace references to enumerate results with either result_pairs
(reference wrapper type) or structured bindings. I did not use
structured bindings everywhere as it wasn't clear to me it would
improve readability.

This is in preparation to the switch to zip semantics which won't
support non-const lvalue reference to elements:
https://reviews.llvm.org/D144503.

I chose to use values instead of const lvalue-refs because MLIR is
biased towards avoiding `const` local variables. This won't degrade
performance because currently `result_pair` is cheap to copy (size_t
+ iterator), and in the future, the enumerator iterator dereference
will return temporaries anyway.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D146006

mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorRewriting.cpp

@@ -377,9 +377,9 @@ public:
       ArrayRef<int64_t> dstShape = dstTp.getShape();
       genReshapeDstShape(loc, rewriter, dstSizes, srcSizes, dstShape,
                          op.getReassociationIndices());
-      for (auto &d : llvm::enumerate(dstShape)) {
-        if (d.value() == ShapedType::kDynamic)
-          dstDynSizes.push_back(dstSizes[d.index()]);
+      for (auto [idx, shape] : llvm::enumerate(dstShape)) {
+        if (shape == ShapedType::kDynamic)
+          dstDynSizes.push_back(dstSizes[idx]);
       }
     }