[libclc] [libclc]: clspv: add a dummy implememtation for mul_hi (PR #134094)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

> > It's a shame we've got such an ugly way of "opting out" of builtins, but I don't know if it's worth coming up with a nicer one.
> 
> Yes, I don't think it's worth it right now.
> 
> Could you please submit that PR?

Sure thing.

One thing I wondered is whether the empty file could be given a comment about why it's intentionally left empty. What do you think?

https://github.com/llvm/llvm-project/pull/134094