[llvm] [SPIRV] Extend lowering of variadic functions (PR #178980)

[Nick Sarnie via llvm-commits]

sarnex wrote:

Those aren't related and fail with any PR that touches SPIRV files since that causes those checks to run, but I'll rebase and hope it's fixed and if not fix it in a separate PR before merging this.

https://github.com/llvm/llvm-project/pull/178980