DenseMap: fix build with clang in C++20 mode

clang was complaing about this code:
llvm/include/llvm/IR/PassManager.h:715:17: error: ISO C++20 considers use of overloaded operator '!=' to be ambiguous despite there being a unique best viable function with non-reversed arguments [-Werror,-Wambiguous-reversed-operator]
      if (IMapI != IsResultInvalidated.end())
          ~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~
llvm/include/llvm/ADT/DenseMap.h:1253:8: note: candidate function with non-reversed arguments
  bool operator!=(const ConstIterator &RHS) const {
       ^
llvm/include/llvm/ADT/DenseMap.h:1246:8: note: ambiguous candidate function with reversed arguments
  bool operator==(const ConstIterator &RHS) const {
       ^

The warning is triggered when the DenseMapIterator (lhs) is not const and so
the == operator is applied to different types on lhs/rhs.
Using a template allows the function to be available for both const/non-const
iterator types and gets rid of the warning

llvm/include/llvm/ADT/DenseMap.h

@@ -1189,8 +1189,6 @@ class DenseMapIterator : DebugEpochBase::HandleBase {
   friend class DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, true>;
   friend class DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, false>;
 
-  using ConstIterator = DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, true>;
-
 public:
   using difference_type = ptrdiff_t;
   using value_type =
@@ -1243,14 +1241,17 @@ class DenseMapIterator : DebugEpochBase::HandleBase {
     return Ptr;
   }
 
-  bool operator==(const ConstIterator &RHS) const {
+  template <typename T>
+  bool operator==(const T &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 {
+
+  template <typename T>
+  bool operator!=(const T &RHS) const {
     assert((!Ptr || isHandleInSync()) && "handle not in sync!");
     assert((!RHS.Ptr || RHS.isHandleInSync()) && "handle not in sync!");
     assert(getEpochAddress() == RHS.getEpochAddress() &&