[llvm] [AA] Take account of C++23's stricter rules for forward declarations (NFC) (PR #109416)

[Jonathan Tanner via llvm-commits]

aDifferentJT wrote:

Also, I don't know the exact procedure (forgive me, this is my first LLVM PR), but it seems from the docs as if some PRs/issues get highlighted for backporting. This is the one issue blocking C++23 adoption in my project, and showed up as soon as I included some high level LLVM headers, I doubt I'm the only one in that boat so it would seem like a good candidate to me.

Thanks for bearing with my inexperience.

https://github.com/llvm/llvm-project/pull/109416