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

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c8ef wrote:

> 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

No problem, I have submitted pull request #94421 for the refactoring.

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