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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Ping, we no longer do this for shaders and the lowering should be the same as the well-tested PTX version.

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