[Flang][OpenMP] NFC: Use ConstructQueue::const_iterator (llvm#102612)

This patch replaces `ConstructQueue::iterator` arguments with
`ConstructQueue::const_iterator` where it's used as a pointer to an
element inside of a `const ConstructQueue &` passed along with it.

Since these functions don't intend to modify the list or any elements in
it, keeping constness consistent between both makes it simpler to work
with.

flang/lib/Lower/OpenMP/Decomposer.cpp

@@ -124,7 +124,7 @@ ConstructQueue buildConstructQueue(
   return constructs;
 }
 
-bool isLastItemInQueue(ConstructQueue::iterator item,
+bool isLastItemInQueue(ConstructQueue::const_iterator item,
                        const ConstructQueue &queue) {
   return std::next(item) == queue.end();
 }

flang/lib/Lower/OpenMP/Decomposer.h

@@ -47,7 +47,7 @@ ConstructQueue buildConstructQueue(mlir::ModuleOp modOp,
                                    llvm::omp::Directive compound,
                                    const List<Clause> &clauses);
 
-bool isLastItemInQueue(ConstructQueue::iterator item,
+bool isLastItemInQueue(ConstructQueue::const_iterator item,
                        const ConstructQueue &queue);
 } // namespace Fortran::lower::omp