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

[Dmitry Sidorov via llvm-commits]

MrSidims 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 :) 

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