[llvm] [ConstantFolding] Fold nextafter/nextafterf/nexttoward/nexttowardf (PR #166250)

[Sayan Sivakumaran via llvm-commits]

sivakusayan wrote:

I'm going to restructure this PR to follow the example of https://github.com/llvm/llvm-project/pull/99611. This PR will first purely add TargetLibraryInfo for `nextafter/nexttoward`, and constant folding will be done in a followup PR.

https://github.com/llvm/llvm-project/pull/166250