[llvm][stlextras] Add const methods to concat_range and enumerator

llvm/include/llvm/ADT/STLExtras.h

using iterator =
                       decltype(std::begin(std::declval<RangeTs &>()))...>;
 
 private:
   std::tuple<RangeTs...> Ranges;
 
   template <size_t... Ns> iterator begin_impl(std::index_sequence<Ns...>) {
     return iterator(std::get<Ns>(Ranges)...);
   }
+  template <size_t... Ns>
+  iterator begin_impl(std::index_sequence<Ns...>) const {
+    return iterator(std::get<Ns>(Ranges)...);
+  }
   template <size_t... Ns> iterator end_impl(std::index_sequence<Ns...>) {
     return iterator(make_range(std::end(std::get<Ns>(Ranges)),
                                std::end(std::get<Ns>(Ranges)))...);
   }
+  template <size_t... Ns> iterator end_impl(std::index_sequence<Ns...>) const {
+    return iterator(make_range(std::end(std::get<Ns>(Ranges)),
+                               std::end(std::get<Ns>(Ranges)))...);
+  }
 
 public:
   concat_range(RangeTs &&... Ranges)
       : Ranges(std::forward<RangeTs>(Ranges)...) {}
 
   iterator begin() { return begin_impl(std::index_sequence_for<RangeTs...>{}); }
+  iterator begin() const {
+    return begin_impl(std::index_sequence_for<RangeTs...>{});
+  }
   iterator end() { return end_impl(std::index_sequence_for<RangeTs...>{}); }
+  iterator end() const {
+    return end_impl(std::index_sequence_for<RangeTs...>{});
+  }
 };
 
 } // end namespace detail
 
 /// Concatenated range across two or more ranges.
 ///
 /// The desired value type must be explicitly specified.
 template <typename ValueT, typename... RangeTs>

 
 template <typename R> class enumerator {
 public:
   explicit enumerator(R &&Range) : TheRange(std::forward<R>(Range)) {}
 
   enumerator_iter<R> begin() {
     return enumerator_iter<R>(0, std::begin(TheRange));
   }
+  enumerator_iter<R> begin() const {
+    return enumerator_iter<R>(0, std::begin(TheRange));
+  }
 
   enumerator_iter<R> end() {
     return enumerator_iter<R>(std::end(TheRange));
   }
+  enumerator_iter<R> end() const {
+    return enumerator_iter<R>(std::end(TheRange));
+  }
 
 private:
   R TheRange;
 };
 
 } // end namespace detail
 
 /// Given an input range, returns a new range whose values are are pair (A,B)