[clang] [SYCL][SPIR-V][Windows] Extend BaseSPIRTargetInfo host-adaptation for Windows support (PR #208196)

[Tom Honermann via cfe-commits]

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tahonermann wrote:

Can you provide more details about how you noticed this? SYCL (at least, I'm not sure about other offload languages that target SPIR-V) doesn't support calls to variadic functions from device code and an appropriate diagnostic should be issued before code gen is reached. A diagnostic is issued in [this case](https://godbolt.org/z/oqjdf6v7G), though only if `f()` is actually called. We do seem to be missing a diagnostic for variadic functions that are explicitly declared as device functions; such functions should be diagnosed as not callable.

https://github.com/llvm/llvm-project/pull/208196