[llvm] [SPIRV] Handle unknown intrinsics (PR #166284)

[Alex Voicu via llvm-commits]

AlexVlx wrote:

> > the user could explicitly specify llvm.
> > I assume this is what you had in mind, and not outright banning ever handling native LLVM intrinsics via this mechanism.
> 
> Yes, I though that we can say: lets ban a stand-alone llvm prefix, but it might be a bit odd. So the idea is to 'create a notion' that native intrinsics should be lowered by backend and not blindly translated to a function call :)

Done-ish, if you'd like to take another peek. I considered adding a warning for the case where the user just provides the option with no prefixes, but that seemed to entail a bit of needless ritual / increased the footprint. It would have been nice to use `ValueRequired`, but that behaves surprisingly / doesn't allow the `=` syntax. We should probably revisit this in the future.


https://github.com/llvm/llvm-project/pull/166284