[libclc] libclc: Replace nextafter implementation (PR #186082)

[Romaric Jodin via cfe-commits]

https://github.com/rjodinchr commented:

I think we don't want to use any floating point operation in the nextafter implementation.

If you really want to avoid reverting, I can test any PR, but I fear that we would just end up being so close to the initial implementation that it would just be simpler to revert first.

https://github.com/llvm/llvm-project/pull/186082