[libclc] [libclc] Add __clc_nan implementation with signed nancode argument (PR #146485)

[Fraser Cormack via cfe-commits]

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

LGTM.

I fixed a couple of minor typos in the description: `Specificatio` -> `Specification` and `no` -> `not`. I hope you don't mind.

https://github.com/llvm/llvm-project/pull/146485