[libclc] [libclc] Bring back fma for clspv (PR #180693)

[Karol Herbst via cfe-commits]

karolherbst wrote:

> My understanding is that once [google/clspv#1533](https://github.com/google/clspv/issues/1533), devices supporting [VK_KHR_shader_fma](https://docs.vulkan.org/refpages/latest/refpages/source/VK_KHR_shader_fma.html) would be able to use `@llvm.fma` through `SPV_KHR_fma`.
> 
> But the fact that clspv supports SPV_KHR_fma does not mean that the device supports it. Thus we will need to be able to handle both paths forever I would say

Fyi, we haven't even started to implement `VK_KHR_shader_fma` inside mesa yet, because it's quite a bit of work. I have it on my todo list, but not sure when I'll have time. So it will take a bit of time until Mesa Vulkan drivers will be able to provide it.

And as I said above, there is still modern hardware that doesn't even support `fma` natively. So I don't think this can be dropped any time soon.

https://github.com/llvm/llvm-project/pull/180693