[llvm] Remove dependence on <ciso646> (PR #73273)

[Jonas Devlieghere via llvm-commits]

https://github.com/JDevlieghere approved this pull request.

> Is there a reason why this never got merged?

Probably because this PR has no reviewers listed. But this LGTM. 

https://github.com/llvm/llvm-project/pull/73273