[llvm] [ADT] Add C++17-style insert_or_assign for DenseMap (PR #94151)

[David Blaikie via llvm-commits]

https://github.com/dwblaikie commented:

Looks great - ideally, the refactoring of the CountCopyAndMove type would be a separate commit - would you be up for sending that in a separate review to be committed ahead of the new insert_or_assign functionality? Otherwise I can probably do that work while still crediting you on the commits

https://github.com/llvm/llvm-project/pull/94151