[ADT] Add public iterator and const_iterator types to iterator_range

[abrachet]

Summary

iterator_range was originally modeled on a proposed std::iterator_range, I never found those papers but boost's iterator_range publicly exposes these types.

[jhenderson]

Just to note: there's a typo in your summary (cosnt_iterator -> const_iterator).

What's the context for this? Why do you need these iterator types defined rather than just using whatever was passed in? (I think I know the answer, but I just want you to explain for people reading this in the future).

[abrachet]

Defining iterator and const_iterator allows it to be used specifically for my use case in D64462 but also in other types I'm sure. Notably many function templates in iterator will not work, std::begin and std::end need one of these types to be publicly defined for example, but one would expect these function templates to work with llvm::iterator_range.

[jhenderson]

Okay, I think this LGTM, but only put this in if this is actually needed in any final version of D64462.

[MaskRay]

llvm/include/llvm/ADT/iterator_range.h

39

You can insert

using iterator = IteratorT;
using const_iterator = IteratorT;

here to avoid adding two access modifiers.

llvm/include/llvm/ADT/iterator_range.h

 namespace llvm {
 
 /// A range adaptor for a pair of iterators.
 ///
 /// This just wraps two iterators into a range-compatible interface. Nothing
 /// fancy at all.
 template <typename IteratorT>
 class iterator_range {
-  IteratorT begin_iterator, end_iterator;
+public:
+  using iterator = IteratorT;
+  using const_iterator = IteratorT;
+
+private:
+  iterator begin_iterator, end_iterator;
 
 public:
   //TODO: Add SFINAE to test that the Container's iterators match the range's
   //      iterators.
   template <typename Container>
   iterator_range(Container &&c)
   //TODO: Consider ADL/non-member begin/end calls.
       : begin_iterator(c.begin()), end_iterator(c.end()) {}
-  iterator_range(IteratorT begin_iterator, IteratorT end_iterator)
+  iterator_range(iterator begin_iterator, iterator end_iterator)
       : begin_iterator(std::move(begin_iterator)),
         end_iterator(std::move(end_iterator)) {}
 
-  IteratorT begin() const { return begin_iterator; }
-  IteratorT end() const { return end_iterator; }
+  iterator begin() const { return begin_iterator; }
+  iterator end() const { return end_iterator; }
 };
 
 /// Convenience function for iterating over sub-ranges.
 ///
 /// This provides a bit of syntactic sugar to make using sub-ranges
 /// in for loops a bit easier. Analogous to std::make_pair().
 template <class T> iterator_range<T> make_range(T x, T y) {
   return iterator_range<T>(std::move(x), std::move(y));