[llvm] [ExpandVariadics] Don't lower unmangled C printf on SPIR-V (PR #206998)

[Paulius Velesko via llvm-commits]

pvelesko wrote:

@jhuber6  

Agreed the printf special-casing is ugly. From my understanding the problem is that in the OpenCL flow printf is implemented by the driver behind the OpenCL.std printf ExtInst, which takes its arguments inline so the call has to reach the backend un-expanded, and there's no device side printf to link in instead (unlike GPU libc on AMDGPU/NVPTX). 

Is there a better way to fix this than matching the name? @sarnex you added the original name matching in #188517, so you may have better insight? 

https://github.com/llvm/llvm-project/pull/206998