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

[Farzon Lotfi via cfe-commits]

https://github.com/farzonl approved this pull request.

Looking specifically to make sure this won’t impact hlsl shaders. Looks like it has sufficient checks for that so LGTM.

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