[clang] Enable unguarded availability diagnostic on instantiated template functions (PR #91699)

[Chris B via cfe-commits]

https://github.com/llvm-beanz approved this pull request.

We've waited a week on this PR to see if anyone from Apple will chime in. On previous PRs we waited weeks and got no response. We've reached out on Discord too.

This PR looks sane to me and it seems to have adequate testing. I think we should move forward and if Apple comes out of the woodwork and has an issue we can revert or move swiftly to address it.

https://github.com/llvm/llvm-project/pull/91699