[llvm] [libc] Add monadic functions to cpp::expected (PR #66523)

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michaelrj-google wrote:

I think that working on a more full implementation sounds like the best path for moving forward. I will warn you that it's going to be more difficult than you expect. There are a lot of strange edge cases that you will run into, especially in the differences between `scanf` and `strto*`. If you're not sure quite how something works, definitely feel free to send me questions or check out the current code. I've tried to annotate the parts that tripped me up when I was writing it.

https://github.com/llvm/llvm-project/pull/66523