[libclc] libclc: generic: add half implementation for erf/erfc (PR #66901)

[Kévin Petit via cfe-commits]

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

Looks infinitely better than the absence of FP16 implementation we currently have. I have not tried to think through rounding and other detailed considerations. The OpenCL CTS currently does not cover FP16 variants of these functions but I believe this is in the works. We can revisit the implementation if the conformance tests reveal issues.

https://github.com/llvm/llvm-project/pull/66901