NFC: Change usage of 'DenseSet' to 'DenseSetImpl' in DenseSetImpl::ConstIterator.

[rriddle]

Summary

Change usage of 'DenseSet' to 'DenseSetImpl' in a friend declaration within DenseSetImpl::ConstIterator. 'ConstIterator' was never updated when DenseSet was split into an impl when adding support for DenseSetImpl.

This fixes build errors on MSVC when forward declaring DenseSet as this friend decl does not declare the template arguments as well.

[lebedev.ri]

This seems to be missing the explanation of the motivation?

[rriddle]

Updated. This was actually a missed change from the split of DenseSet here(https://reviews.llvm.org/D25628) that results in build errors on MSVC when forward declaring DenseSet. This friend decl does not forward declare the template arguments as well, which creates a mismatch.

[jpienaar]

This make the declaration of ConstIterator and Iterator equivalent (line 107), so SGTM.

llvm/trunk/include/llvm/ADT/DenseSet.h

public:
     Iterator& operator++() { ++I; return *this; }
     Iterator operator++(int) { auto T = *this; ++I; return T; }
     bool operator==(const ConstIterator& X) const { return I == X.I; }
     bool operator!=(const ConstIterator& X) const { return I != X.I; }
   };
 
   class ConstIterator {
     typename MapTy::const_iterator I;
-    friend class DenseSet;
+    friend class DenseSetImpl;
     friend class Iterator;
 
   public:
     using difference_type = typename MapTy::const_iterator::difference_type;
     using value_type = ValueT;
     using pointer = const value_type *;
     using reference = const value_type &;
     using iterator_category = std::forward_iterator_tag;