[libclc] libclc: increase fp16 support (PR #98149)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

> Also, just leave that thought here: A next step might be to use `native` functions instead (when available). They should provide enough precision for half, but be more performance. Also as the accuracy is undefined for those functions, maybe it should be an opt-in option in the libclc (clspv would enable it for sure).

Yeah, it would definitely need to be opt-in. Ideally we'd also implement the `half` builtins properly so they're not going through `float` and are as fast as they can be, but then _also_ allow native functions if users so wish. But that involves maths, haha.

https://github.com/llvm/llvm-project/pull/98149