[clang] [llvm] [SPIR-V] Enable variadic function lowering for the SPIR-V target (PR #175076)

[Nick Sarnie via cfe-commits]

sarnex wrote:

> Is there an easy way to detect this? Like a module flag?

Maybe this?

```
if(!SPIRVTargetMachine.getSubtargetImpl()->isShader())
```

https://github.com/llvm/llvm-project/pull/175076