[llvm] [ADT] Add [[nodiscard]] to AP*.h (NFC) (PR #161776)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

I have similar feelings -- this seems quite verbose. Having `[[nodiscard]]` on core data structures etc. makes sense because of how widely they are used. For AP*, it may fall and the same bucket, but I don't think all of these functions are likely to be misused. Could we identify a subset that seems prone to misuse and start with that?

https://github.com/llvm/llvm-project/pull/161776