Fix DenseMapIterator for C++20 compliance

[jcelerier]

Summary

C++20 has stricter requirements regarding comparison operators :
the conversion-inducing (in the non-const_iterator case) operator==(const DenseMapIterator&) const;
creates ambiguousness with operator!=(const DenseMapIterator&) const;.

llvm/include/llvm/ADT/DenseMap.h

public:
   pointer operator->() const {
     assert(isHandleInSync() && "invalid iterator access!");
     assert(Ptr != End && "dereferencing end() iterator");
     if (shouldReverseIterate<KeyT>())
       return &(Ptr[-1]);
     return Ptr;
   }
 
-  bool operator==(const ConstIterator &RHS) const {
+  bool operator==(const DenseMapIterator &RHS) const {
     assert((!Ptr || isHandleInSync()) && "handle not in sync!");
     assert((!RHS.Ptr || RHS.isHandleInSync()) && "handle not in sync!");
     assert(getEpochAddress() == RHS.getEpochAddress() &&
            "comparing incomparable iterators!");
     return Ptr == RHS.Ptr;
   }
-  bool operator!=(const ConstIterator &RHS) const {
+  bool operator!=(const DenseMapIterator &RHS) const {
     assert((!Ptr || isHandleInSync()) && "handle not in sync!");
     assert((!RHS.Ptr || RHS.isHandleInSync()) && "handle not in sync!");
     assert(getEpochAddress() == RHS.getEpochAddress() &&
            "comparing incomparable iterators!");
     return Ptr != RHS.Ptr;
   }
 
   inline DenseMapIterator& operator++() {  // Preincrement