clean up std::iterator in LibcBenchmark.h

[SchrodingerZhu]

Remove std::iterator usage in LibcBenchmark.h

[SchrodingerZhu]

@gchatelet Would you mind merging this?

libc/benchmarks/LibcBenchmark.h

 
 public:
   using value_type = T;
   using reference = T &;
   using const_reference = const T &;
   using difference_type = ssize_t;
   using size_type = size_t;
 
-  class const_iterator
-      : public std::iterator<std::input_iterator_tag, T, ssize_t> {
+  class const_iterator {
+    using iterator_category = std::input_iterator_tag;
     llvm::ArrayRef<T> Array;
     size_t Index;
     size_t Offset;
 
   public:
     explicit const_iterator(llvm::ArrayRef<T> Array, size_t Index = 0)
         : Array(Array), Index(Index), Offset(Index % Array.size()) {}
     const_iterator &operator++() {