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